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How to import CSS files to React JS application?

I am creating an application, created multiple CSS files but those are not able to import.
I tried by installing css-loader and style-loader. But it doesn't help
Please look below picture, I have Burger.css file. but it not visible to import

VS Code by default doesn't autocomplete all file types. If you write import ./Burger/Burger.css or import ./Burger/BurgerIngredient/BurgerIngredient.css in your Layout.js file, your css files will be loaded fine.
If you want to use autocomplete for all files in VS Code, you can use this extension.
https://marketplace.visualstudio.com/items?itemName=ionutvmi.path-autocomplete
Without Extension
With Extension

Ok since you confirmed that your ./Layout.css import does not work, you should not name your import when it's about a css file. Doing :
import './myFile.css';
is enough to apply your css.
Note that css is global in React, so you might want to use styled component or encapsulate your css to prevent side effect on other components).
Now, if you really want to add style the way you tried to, there is the style attribute which accepts an object. So you could do:
<div style={{backgroundColor: 'red'}}>Hello World !</div>
Note that the css properties are written in camelCase.

Your file structure appears to me to be...
-Burger
--BurgerIngredient
----Burger.css
--Layout
----Layout.css
Your primary application, from what it appears here, is in /Burger. Considering that, when you type import, you see the dropdown appear, showing helpful suggestions, and it lists...
-../Burger/BurgerIngredient/...
As a possible, valid location. In that case, why don't we try importing css, by typing out...
import burgercss from '../Burger/BurgerIngredient/Burger.css';
Note, for instance, Burger.css wouldn't show up until you select BurgerIngredient, because that's its conntaining folder.

Bootstrap cdn overrides my custom css styles in react js

I have designed a web page in ReactJS using CSS. For one of the component I have used Bootstrap for styling. I garbed the latest bootstrap cdn link and pasted it in "index.html" file. When I use that bootstrap link, it overrides few of my custom styles. Each component of my project has a separate CSS and JS file. Please suggest any solution, where should I put that bootstrap link in my project so that it doesn't override my custom css styles. Thanks.

It's smell like wrong css include order. You may check it in browser. To resolve this, you can go through few ways:
Try to include bootstrap css as very first link in head section (after meta).
Try to include your css via import in your index.js file.
Second way should be look like this:
import "https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/css/bootstrap.min.css";
import "./styles.css";

It’s very simple. Link your custom CSS file just below the bootstrap.css and override the class in your custom.css file.
You can rely on descendant selectors, ID and ‘!important’ notation together to override the styles but that is against the CSS best practices. You can use them only when it is absolutely necessary.
You can simply use this method. Follow the following articles. I hope you help this.
Override Bootstrap CSS styles
CSS Overrides

AngularJS custom bootstrap CSS not working at component level

I decided to convert a landing page to an AngularJS website, because I needed to add an admin section. To separate the website section from the admin section, I created two modules: website and admin.
The original website has been made with Bootstrap 3, and has a style.css that is custom CSS for all the Bootstrap and the website in general.
On the Angular version, I can load the website properly after I installed Bootstrap 3, and in the root-level style.css I do the following :
#import './app/website/assets/css/style.css';
The issue is that I don't want this CSS to be loaded for the full website (website + admin). With this configuration, the admin section is also affected by the CSS.
The import only works if it is in style.css. If I move the import to the website module in the root component.css styles won't load at all.
I know it must have something to do something with style scoping and ng-deep.
EDIT: The only way I can have the website load properly with the CSS imports within its own module is :
encapsulation: ViewEncapsulation.None

As of right now, there is no way to import css at the module level. Global styles are applied globally, and the default ViewEncapsulation makes it so that component specific styles don't bleed out to the rest of the app.
If I move the import to the website module in the root component.css
styles won't load at all.
Importing that css at the modules root component only applies the styles to that one component. I also wouldnt look too hard at ng-deep as it is / will be deprecated https://angular.io/guide/component-styles#deprecated-deep--and-ng-deep
Without knowing how many components are in WebsiteModule or what styles.css looks like, I'll present two options.
1) Rename styles.css (could get confusing since it's not going to be global anymore), import it in each of the components in WebsiteModule.
If that turns out to be too tedious (one bazillion components in WebsiteModule), then
2) I would take a good hard look at the styles.css in question, and see what styles should be applied globally.
Turning off ViewEncapsulation should be a last resort IMO.

How can I style React-Table?

I am using react-table to create a filterable table, however I wanted to style it the way I want. Should I change it directly inside node_modules application or should I create my own CSS file and override inspecting the element on the fly?

For React-Table v6, you can basically do it in 2 ways if you want to use css files.
Start with the css file from the npm package and override pieces you want to change
Start with your own css file and style everything that needs to be styled
Ad 1:
import "react-table/react-table.css"
import "./your_own_styles.(s)css"
Ad 2:
import "./your_own_styles.(s)css"
To get an idea what you can use in your own css file you can have a look at this gist: https://gist.github.com/sghall/380aa1055bcad4db22d1fc258d7f09d0
Or start by copying the react-table/react-table.css from the react-table npm package (in your node_modules).

Definitely do not change the node_modules. You can either style inline or attach styles.
https://reactjs.org/docs/faq-styling.html

Definitely override the CSS classes with your own file. That's common practice for styling components from libraries.

import css using webpack in react

I was from angularjs, now picking up react. Even I was using angular 1.x which is already component based, but it still has template. But in react the file structure and the way we use to code front end has changed, like instead of spiting files by pages, u make files by component now. It promotes reusability but does that means how we apply the css also changed?
I saw this import { navBar } from 'styles/navbar.css' in navBar.jsx. Hmm how does css work together with JSX? doest navBar css load that file? What webpack plugin is needed for that? does it come from default? I'm using react-create-app by facebook so I didn't know much about config.

You use css-loader and style-loader to include CSS files in your webpack bundle. By default it generates some JavaScript code that creates a style element with the contents of the imported CSS file and appends it to the head element in your index.html.
So you can definitely use external CSS files to style your React components, just make sure that every CSS class is properly namespaced to avoid naming conflicts with the classes of other components.
For example you could adopt the BEM naming scheme. If your component is called NavBar, then the root element of that component might have a className of x-nav-bar (the x prefix is there to avoid clashing with frameworks like bootstrap), and all child elements, if they need to be styled, will then have class names like x-nav-bar__${childName}.

This kind of import { navBar } from 'styles/navbar.css' is not relevant to JSX but to css-loader. This is a webpack loader that handles css, and it supports cssModules, that allows you to encapsulate selector names in order to avoid css leaks.
So, shortly, that import exposes an object with mapping between your selector to unique string (usually an a hash).
For example:
// styles.css
.foo {
color: red;
}
.bar {
color: green;
}
// Component.jsx
import styles from './styles.css';
console.log(styles);
/* This will print something like
{
foo: '_h234jh',
bar: '_234m23'
}
*/