$ is not defined jQuery 3.5.1 window on load

Question

I have jQuery as a script source before my custom site.js file.

<script src="/Scripts/jquery-3.5.1.js"></script>
<script src="/Scripts/site.js"></script>

In site.js I have the following:

$(window).on('load', function () {
    console.log('test');
});

The following is the output from the console:

VM318 site.js:142 Uncaught ReferenceError: $ is not defined
    at VM318 site.js:1:1
(anonymous) @ VM318 site.js:1
site.js:2 test

I am not sure what I am missing here? I have written a function to test jQuery and it works in site.js:

function testFunc() {
    let test = $('#test');
}