How to make Javascript (raining matrix code) as a background of the website

The problem I encounter is that I dont know how to overlay a text, image, and video on a javascript. I tried to find it on youtube and google but I find no answer. Can somebody help me? Thanks!

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="./styles.css">
    <title></title>
</head>
<body style="background-color: black">
  

    <canvas id="Matrix" style="width: 100%; height: 100%;"></canvas>
    <script src="./index.js"></script>

  


</body>
</html>
html {
    background: black;
    height: 100%;
    overflow: hidden;
}

body {
    margin: 0;
    padding: 0;
    height: 100%;
}
const canvas = document.getElementById('Matrix');
const context = canvas.getContext('2d');

canvas.width = window.innerWidth;
canvas.height = window.innerHeight;

const katakana = 'アァカサタナハマヤャラワガザダバパイィキシチニヒミリヰギジヂビピウゥクスツヌフムユュルグズブヅプエェケセテネヘメレヱゲゼデベペオォコソトノホモヨョロヲゴゾドボポヴッン';
const latin = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
const nums = '0123456789';

const alphabet = katakana + latin + nums;

const fontSize = 16;
const columns = canvas.width/fontSize;

const rainDrops = [];

for( let x = 0; x < columns; x++ ) {
    rainDrops[x] = 1;
}

const draw = () => {
    context.fillStyle = 'rgba(0, 0, 0, 0.05)';
    context.fillRect(0, 0, canvas.width, canvas.height);
    
    context.fillStyle = '#0F0';
    context.font = fontSize + 'px monospace';

    for(let i = 0; i < rainDrops.length; i++)
    {
        const text = alphabet.charAt(Math.floor(Math.random() * alphabet.length));
        context.fillText(text, i*fontSize, rainDrops[i]*fontSize);
        
        if(rainDrops[i]*fontSize > canvas.height && Math.random() > 0.975){
            rainDrops[i] = 0;
        }
        rainDrops[i]++;
    }
};

setInterval(draw, 30);

I am expecting the javascript to be the background of my entire website while only using HTML,CSS, and JS.