React - Module not found: Can't resolve '#bootstrap-styled/v4' - reactjs

Question

I wonder why React can't find '#bootstrap-styled/v4'.
At first, I didn't eject. Because it's my first React project. I want simple.
yarn add '#bootstrap-styled/v4'
So this package installed in 'Project/node_modules/#bootstrap-styled/v4'.
And package.json file here.
# package.json
{
"name": "front",
"version": "0.1.0",
"private": true,
"dependencies": {
"#bootstrap-styled/v4": "bootstrap-styled/v4",
"#testing-library/jest-dom": "^4.2.4",
"#testing-library/react": "^9.3.2",
"#testing-library/user-event": "^7.1.2",
"react": "^16.13.1",
"react-dom": "^16.13.1",
"react-scripts": "3.4.1",
"styled-components": "^5.1.1"
},
"scripts": {
"start": "cross-env NODE_PATH=src react-scripts start",
"build": "cross-env NODE_PATH=src react-scripts build",
"test": "react-scripts test",
"eject": "react-scripts eject"
},
"eslintConfig": {
"extends": "react-app"
},
"browserslist": {
"production": [
">0.2%",
"not dead",
"not op_mini all"
],
"development": [
"last 1 chrome version",
"last 1 firefox version",
"last 1 safari version"
]
},
"devDependencies": {
"cross-env": "^7.0.2"
}
}
# src/index.js
import React from 'react';
import ReactDOM from 'react-dom';
import 'index.css';
import BaseLayout from 'BaseLayout';
import * as serviceWorker from 'serviceWorker';
ReactDOM.render(
<React.StrictMode>
<BaseLayout />
</React.StrictMode>,
document.getElementById('root')
);
serviceWorker.unregister();
# src/BaseLayout.js
import React, { useState } from 'react';
import {
Nav,
NavItem,
NavLink,
NavDropdown,
DropdownToggle,
DropdownMenu,
DropdownItem,
} from '#bootstrap-styled/v4';
// root component
function BaseLayout() {
const [isOpen, setIsOpen] = useState(false);
return (
<Nav>
<NavItem>
<NavLink href="">Home</NavLink>
</NavItem>
<NavDropdown isOpen={isOpen} toggle={() => setIsOpen(true)}>
<DropdownToggle>
General
</DropdownToggle>
<DropdownMenu>
<DropdownItem>1</DropdownItem>
<DropdownItem>2</DropdownItem>
<DropdownItem>3</DropdownItem>
</DropdownMenu>
</NavDropdown>
</Nav>
);
}
export default BaseLayout;
I ran the server, error displayed.
Module not found: Can't resolve '#bootstrap-styled/v4' in 'D:\Project\src'
I have a few questions here.
I heard if you don't eject, React project will find module in src only. Is that right?
1-1. If correct, I don't make sense. Then why does node_modules exist? It would be right to go inside src, right?
What mean '#'? This isn't a scoped package, is it? Cause scoped package's structure is #scope/packageName. '#bootstrap-styled/v4' does not apply. (At https://bootstrap-styled.github.io/bootstrap-styled/, you can see their packages.)