I wanted to put this as a short comment, but it get’s unreadable. I also don’t go into segwit.
I am not clear on your question: are you looking how to use the OpenSSL library for iOS (then this might not be the right forum - review of parameters to an OpenSSL library is not bitcoin specific...), or more at the conceptual steps to come to an address? Then read on:-)
Searching for OpenSSL and keys in the forum reveals lots of hints on how to use OpenSSL at the command line. I put here some links, that I find useful for creation and conversion of keys:
Bitcoin Address generation
What are steps 4-9 in bitcoin address generation for?
Then you‘ll have to go some additional steps, as described in bitcoin wiki.
And finally, if you want to check the addresses, this is a useful page:
Let us know how it goes, and see where we can bring you to your goal!