I’ve been seeing people upload videos from YouTube and I don’t know how. Do you just put the link in or do you have to pay to download then put that file here?

Use something like youtube-dl (or any of the various free apps that use youtube-dl under the hood) to download a high-quality WEBM version of the video without reconverting it, and you can upload that here. Just be wary of the filesize limits, quality restrictions, and other such standard rules for uploads.