I’m in the process of trying to get buildbot to build RetroArch and iOS cores automatically. But in the meantime, I can manually upload them to buildbot for download.
Link to the IPAs:
You can use Cydia Impactor to install with apple ID, or re-sign it with a resigning tool like iResign.
The app has the latest versions of the cores preinstalled. There are some new ones supported for iOS - most notably MAME 0.212, Mesen and Mesen-S (if you’re into Mesen).