Which is a better response to “How are you”?

It's really just a matter of preference since they're both understandable!

Both 100% correct! The what and how are interchangeable in this context

Is "yourself" actually correct here? Why not just use "you"? It's not clear to me why a reflexive pronoun would be needed (though I've definitely heard people use one in similar exchanges).

Either is fine. I personally say "good, and you/yourself?"

Couple choices: Good, and yourself? Or Good, and you?

« I’m fine thank you. And you ? » Normally it’s better to use « fine » than « good » with this answer in standard English (for English second language speakers/learners). But I think it is an old thing and now native speakers use both (fine and good).

"What about you," not "What about yourself" ("how about you" also works, but "what about yourself" sounds wrong)

One of many British answer versions is "Yeah good thanks how are you?"

First one, because you essentially ask them in return the same question they asked you, which starts with HOW. It connects better. It feels more conversational. The other question is OK, but it’s not the same question. WHAT does not equal HOW. So it doesn’t have that same parallel feel. You’ll be fine either way, but I have a definite preference for the connotation of using using HOW. ☺️ It feels more elegant to me.

I have been using “and you?” lately 😊

They’re both fine.

Can someone tell me why we use "yourself" instead of "you?"

They're both fine! Often I'll even just say "Good, you?" Because no one actually cares what your response is.