On centralized platforms this all happens on the server, transparent to the user. Anything you upload is compressed and converted to the correct formats. On Nostr it's not universal. That's why a lot of videos don't load for you, and possibly some other users can't view yours.

Yeah unfortunately Nostr requires a bit more knowledge and effort than centralized services. That's why there's so many videos with comments saying "I can't see it" or "I can't hear it", or your video just doesn't get much engagement and you don't even know, because half the people can't even view it properly.