Well, looks like you're just interested on the code right, so I'll just drop the code in here.
Obviously, feels free to move code around and make sure you use the syntax that does make sense to you

This is the context

CoinsContext.js

import React, { createContext, useContext, useState, useEffect } from "react";

import { getCoin } from "@api";



const CoinsContext = createContext({});



export const CoinsContextProvider = ({ children }) => {

  const [selectedCoin, setSelectedCoin] = useState(null);

  const [coins, setCoins] = useState([]);



  useEffect(() => {

    const fetchAPI = async () => {

      const data = await getCoin();

      setCoins(data);

    };

    fetchAPI();

  }, []);



  return (

    <CoinsContext.Provider value={{ selectedCoin, setSelectedCoin, coins }}>

      {children}

    </CoinsContext.Provider>

  );

};



export const useCoins = () => useContext(CoinsContext);

To use it, you need to wrap up all the pages that are gonna use the context with it, for example, since you want Details.js and Company.js to use it, you will need to wrap both pages with it.
I'll just wrap the entire App, which means the entire app can use it, but feel free to do what you want.

Would be something like this:

index.js

import React from "react";

import ReactDOM from "react-dom";

import App from "./App";



import { CoinsContextProvider } from "./CoinsContext";



ReactDOM.render(

  <React.StrictMode>

    <CoinsContextProvider>

      <App />

    </CoinsContextProvider>

  </React.StrictMode>,

  document.getElementById("root")

);

Now we have the CoinsContext set up, we can use it on your components

Company.js

import React from "react";

import { Col, Image, Row } from "react-bootstrap";

import "./Company.scss";



// *spinner

import Loader from "./Loader";

import { useCoins } from "./CoinsContext";



const Company = () => {

  const { coins, selectedCoin, setSelectedCoin } = useCoins();



  return (

    <>

      {coins.length > 0 ? (

        coins.map((coin) => (

          <Row

            className={

              selectedCoin === coin.id

                ? "p-2 pe-3 border-top d-flex align-items-center company-list-single-active"

                : "p-2 border-top d-flex align-items-center company-list-single"

            }

            onClick={() => {

              setSelectedCoin(coin.id);

              // console.log(coin.id);

              // console.log(coin.name);

            }}

            key={coin.id}

          >

            <Col xxl="2" xl="2" lg="3" md="3" sm="2" xs="2">

              <Image

                src={coin.image}

                alt={coin.name}

                className="coin-image mx-2"

                fluid

              />

            </Col>

            <Col>

              <span>{coin.name}</span>

            </Col>

          </Row>

        ))

      ) : (

        <Loader />

      )}

    </>

  );

};

export default Company;

Details.js

import React, { useState, useEffect } from "react";

import axios from "axios";



import { useCoins } from "./CoinsContext";



const Details = () => {

  const { selectedCoin, coins, setSelectedCoin } = useCoins();

  

  const [data, setData] = useState({

    name: "",

    id: "",

  });





  const apiDetails = async () => {

    await axios

      .get(`https://api.coingecko.com/api/v3/coins/${"ethereum"}`)

      .then((r) => {

        // console.log(response);

        setData({

          name: r.data.name,

          id: r.data.id,

        });

        return setData;

      });

  };



  useEffect(() => {

    (async () => {

      const response = await apiDetails();

      setData({

        name: response.data.name,

        id: response.data.id,

      });

    })();

  }, []);



  return (

    <div>

      <h1>{data.name}</h1>

      <h1>{data.id}</h1>

    </div>

  );

};



export default Details;

And we're done! Now you are not just sharing selectedCoin between your components, but you're also putting all coins fetching logic in your context, which overall is the right thing to do