Trying to understand seed phrase

Can someone please tell me why 12 and 24 word seed phrases are “legacy” and 20 word ones are better or worse? Wouldn’t more words mean more combinations? Also is there a better security than that? I’m also trying to understand coldwallets better and trying to explain it to friends scares them with losing all their bitcoin if they lose their coldwallet or seedphrase it’s gone forever but I’m trying to say you are sovereign when you have control rather than someone else owning your assets. I think eventually big companies will hold bitcoin for people because they don’t trust themselves and get bogged in fees. What’s anyone’s thought on these things?