I wonder how to use tag correctly tho

If it's fandom: fandom name, character name, ship name if relevant, nsfw if relevant. If not fandom: what it's about, what it is, or who's involved. Cats, humor, sports, photography, knitting, etc. You can make personal tags too, like I might use 'aewinwrites' to keep all my fic in one place.

for getting it to show up in feeds for different shows, games etc. I think you just need to have the title or a common acronym/abbreviation beyond that there's no need to think too hard about it, you can just tag whatever (ideally something relevant)