Display fraction of a dollar value

Example at Find Satoshi's Chest | A Real Treasure Hunt for .021 Bitcoin

Currently displaying $ value of 1 bitcoin. I would like to show a fraction of this number - namely 0.021 at all times. I understand the math, but I copied this code from another tutorial, I don’t know how to edit the script to make this work.

Javascript here

// SELECT ELEMENTS
const usd = document.querySelector('.usd');

// DEFINE URL WHERE TO FETCH DATA
const url = 'https://api.coindesk.com/v1/bpi/currentprice.json';
//usd.innerHTML = 'current worth...'


// FETCH BITCOIN PRICE
const fetchDataFn = async ()=>{
    try{
    let bitcoinFetch = await fetch(url);
    let bitcoinPrice = await bitcoinFetch.json();
    setTimeout(()=>{
        // SET NEW INNER HTML
        usd.innerHTML = `${bitcoinPrice.bpi.USD.rate}`;
    }, 1000); // SHOW AFTER ONE SECOND
    }catch(error){
        if(error) console.log(error);
    };
};
// Run fetch function ON START
fetchDataFn();

// UPDATE VALUES EVERY 10 SECONDS
setInterval(()=>{
    fetchDataFn();
},10000);