React -router doesn't add component to the dom - reactjs

Question

I have Quotes component which contains list of quotes. When I click individual list element, react router changes the url dynamically and opens it in full page which is Full_Screen_Quotes then again inside the Full_Screen_Quotes I want to add Comments section on button click with the help of react router, but it does not work for the reason I do not know. What might be the reason?
import { useParams } from "react-router-dom/cjs/react-router-dom.min"
import Comments from "./Comments"
import classes from '../css/Full_Screen_Quote.module.css'
import db from "./db"
import { useState } from "react"
import { Router,Route } from "react-router-dom/cjs/react-router-dom.min"
import { Link } from "react-router-dom"
import { Switch } from "react-router-dom/cjs/react-router-dom.min"
const Full_Screen_Quotes = ()=>{
const params = useParams()
return(
<>
<div className={classes.quote}>
<h1>{params.quoteId} </h1>
<h4>{params.authorId}</h4>
<Link to = {`/Allquotes/${params.authorId}/${params.quoteId}/comments`}> // Link that adds comment section
<button>ADD</button>
</Link>
</div>
<Route path ={`/Allquotes/${params.authorId}/${params.quoteId}/comments` } exact > //Comment section path
<Comments></Comments>
</Route>
</>
)
}
export default Full_Screen_Quotes
App js
import logo from './logo.svg';
import './App.css';
import {Route} from 'react-router-dom'
import {Switch} from 'react-router-dom'
import {Link} from'react-router-dom'
import MainHeader from '../src/components/MainHeader'
import Quotes from '../src/components/Quotes'
import AddQuotes from './components/AddQuotes';
import Welcome from './components/Welcome'
import {useParams} from 'react-router-dom'
import ListElement from './components/ListElement';
import Full_Screen_Quote from '../src/components/Full_Screen_Quote'
import {Redirect} from 'react-router-dom'
import Home from '../src/components/Home'
import NotFound from './NotFound';
import {useState,useCallback,useMemo} from 'react'
import { Prompt } from 'react-router-dom/cjs/react-router-dom.min'
import Comments from './components/Comments';
function App() {
const [is_focused,set_is_focused] = useState(false)
return (
<div>
<Switch>
<Route path ='/Welcome/' exact>
<Welcome/>
</Route>
<Route path ='/Allquotes/' exact>
<Quotes />
</Route>
<Route path ='/Addquote/' exact>
<AddQuotes set_is_focused = {set_is_focused} is_focused={is_focused} />
</Route>
<Route path ='/Allquotes/:authorId/:quoteId' exact>
<Full_Screen_Quote />
</Route>
</Switch>
</div>
)
}
export default App;
Comments js
import { useState } from "react"
import db from '../components/db'
import { useRef } from "react"
const Comments = (props)=>{
return (
<div style={{display:'flex',justifyContent:'center'}}>
<div> <textarea style={{transition:'1s all'}}></textarea> </div>
</div>
)}
export default Comments

Answer 1

Your root router/Switch is exactly matching URL paths, and in the case of rendering the Full_Screen_Quote component it only exactly matches up to '/Allquotes/:authorId/:quoteId'
<Route path ='/Allquotes/:authorId/:quoteId' exact>
<Full_Screen_Quote />
</Route>
As soon as the path becomes "/Allquotes/someAuthorId/someQuoteId}/comments" it no longer matches exactly and your Full_Screen_Quote component is unmounted, thus unmounting the nested Route you want to render for the Comments component.
In 99.99% for use cases there's really no need to use the exact prop in conjunction with the Switch since you can order the rendered routes in inverse order of path specificity so matching can work properly. Using the exact prop also necessarily precludes the further matching of any nested routes.
You've a couple options:
Order the routes in the Switch in descending path specificity order and remove the exact prop. This allows the nested route in Full_Screen_Quote to be eventually matched and rendered.
<Switch>
<Route path='/Welcome/'>
<Welcome />
</Route>
<Route path='/Allquotes/:authorId/:quoteId'>
<Full_Screen_Quote />
</Route>
<Route path='/Allquotes/'>
<Quotes />
</Route>
<Route path='/Addquote/'>
<AddQuotes
set_is_focused={set_is_focused}
is_focused={is_focused}
/>
</Route>
</Switch>
...
const Full_Screen_Quotes = () => {
const { path, url } = useRouteMatch();
const params = useParams();
return (
<>
<div className={classes.quote}>
<h1>{params.quoteId} </h1>
<h4>{params.authorId}</h4>
<Link to={`/${url}/comments`}>
<button>ADD</button>
</Link>
</div>
<Route path={`${path}/comments`}>
<Comments />
</Route>
</>
);
};
Move the nested route out to the main router/Switch and order the routes in descending path specificity order and remove the exact prop.
<Switch>
<Route path='/Welcome/'>
<Welcome />
</Route>
<Route path='/Allquotes/:authorId/:quoteId/comments'>
<Comments />
</Route>
<Route path='/Allquotes/:authorId/:quoteId'>
<Full_Screen_Quote />
</Route>
<Route path='/Allquotes/'>
<Quotes />
</Route>
<Route path='/Addquote/'>
<AddQuotes
set_is_focused={set_is_focused}
is_focused={is_focused}
/>
</Route>
</Switch>
...
const Full_Screen_Quotes = () => {
const { url } = useRouteMatch();
const params = useParams();
return (
<div className={classes.quote}>
<h1>{params.quoteId} </h1>
<h4>{params.authorId}</h4>
<Link to={`/${url}/comments`}>
<button>ADD</button>
</Link>
</div>
);
};

Answer 2

wrap the app.js in a router
import {BrowserRouter as Router} from 'react-router-dom'
then
<Router>
<Switch>
<Route path ='/Welcome/' exact>
<Welcome/>
</Route>
<Route path ='/Allquotes/' exact>
<Quotes />
</Route>
<Route path ='/Addquote/' exact>
<AddQuotes set_is_focused = {set_is_focused} is_focused={is_focused} />
</Route>
<Route path ='/Allquotes/:authorId/:quoteId' exact>
<Full_Screen_Quote />
</Route>
</Switch>
</Router>