Okay, if this isn't your first rodeo with webdev then I suggest that you stop reading this post, because it won't add much to what you already know. If you're new or have only heard of webdev then you've come to the right place. Here are 5 concepts that'll make the web less confusing.
What's a document. Everything that you see on the web is a document. All of it, videos, pictures, and text are all visible because they're displayed through the document.
So Youtube is a document?
The client-side is your computer. That's all. It means that there is no interaction with a server. It means vanilla js, no node, no deno, no server runtime.
HTTP stands for Hyper-Text Transfer Protocol. Hyper-Text in directly refers to markup langs, like html, and Transfer Protocol means that it's the procedure to move the markup from one place to another. In this case from a server to your client(computer, or mobile device).
That's that. Hope you learned something.