Are Security Keys on iPhone or Mac an optional way to compete Apple ID 2FA or a further step after 2FA text-phone-trusted device codes?

Read all I can find on Apple support on Security Keys. Before I buy a 2nd one to satisfy Apple’s 2 or more requirement, I’m puzzled whether Security Keys are an additional requirement, to and no top of the usual 2FA authentication. That is, if you enable a security key, go to sign in to Apple ID, you must complete, UN and PW, then 2FA codes or trusted device, and still use the security key. The security key becomes an additional requirement, after one of the 2FA screens?

I thought security keys were a way to accomplish 2FA. I have an RSA to sign into online banking. I give UN+PW, the EITHER the code from the RSA or from text or call. Other websites do the same.

Important to me b/c I’m out of country a lot and often can’t get codes with texts or calls. security keys allow me to complete 2FA. As I read it, with Apple, I’d still be stuck at 2FA screen. The security key doesn’t help me, correct?