@TohruAdachi
There's a thread somewhere on archiving, but I can't find it so w/e here's a quick brief. This was originally going to be a PM but odds are at least one other person who doesn't know how to archive is going to read this thread.
For archiving webpages use the
wayback machine and archive.fo, archive.md, archive.md, or whatever their domain of the week is. There's a nifty browser addon called "Resurrect Pages" that can partially automate this. The Wayback Machine is sensitive to takedown requests and archive.whatever has a low bandwidth cap, hates brave, and will sometimes force a captcha on you so it's not as good as it once was. Ideally everything should be archived to both and then a copy of the page should be saved and stuck in a zip or tar.gz file via wget, curl, or right-click > Save Page As for full coverage, but people are lazy.
For readability it's always a good choice to take screencaps and attach them directly to posts, but always ensure there's an archive of the webpage itself somewhere because screenshots can be disputed or repudiated, archives can't so easily. If you're on *nix, pretty much every distro has an easy click and drag screenshot tool like gnome-screenshot or whatever your de is that you can use to name and save in one click. If you still haven't left the botnet I'm sure there's some gui tool for screenshots that does the same thing, but there's always PrtScrn and mspaint.
For videos,
youtube-dl is your friend. Despite the name, it works with a lot of web 2.0 video "services". For yt videos you can also download through
freetube or an
invidious mirror. There's a more up-to-date list of mirrors on github somewhere, but I'm too lazy to search for it right now. I don't do the whole discord/instagram/twitter thing so someone else can comment on that but I think dear leader is working on tools for these already.
E: always archive old.reddit.com , rather than the regular domain. It has more html and less javascript bullshit which means archives will show more and are less likely to break.