how to stop users to add something like this as username or first name? & what's this called?

These are mostly obscure unicode characters, you can enforce only alphanumeric usernames while blocking unicode, but for display names, preventing unicode may bother a lot of users who have legit accented letters in their names.

Zalgo, as they are called https://en.m.wikipedia.org/wiki/Zalgo_text

Got it, even NFT marketplaces are allowing this kind of usernames, not sure why but it also giving accessibility issues