Technology does get better… and when you update it regularly you would be informed of how it was changing and what to do to stay compatible. Unfortunately, jumping 6 versions and 7 years from Yosemite to Big Sur is quite a change. Apple has dropped support for Quicktime 7, all 32 bit applications, and all 32 bit codecs. I’m guessing those are some of the ones you are missing. (This is why I stopped at Mojave because this started with Catalina)
That article has a list of video formats still supported, and the ones that are no longer supported. If you still have your Yosemite system you can probably still do the conversions they discuss. I’m pretty sure that you cannot convert them on the new system because the old codecs simply don’t exist for it.