I signed up with email — can I switch to Google or Apple later (or the other way round)?

Yes, and you do not lose anything by doing it. If you sign in with Google or Apple using the same email address as your existing Creta account, the two are linked automatically. It stays one account — same nickname, same purchases, same points, same wallet — and from then on either method signs you in.

Linking only happens when both sides have confirmed the address: the provider says the email is verified, and your Creta account has verified it too. That is what stops someone else claiming your account by signing up on your address.

If the provider account uses a different address, you get a second, separate Creta account instead. Accounts cannot be merged afterwards, so sign in with the address you already use.