I've been building and testing my iOS app just for fun via Linux only.

Surprisingly, it's very easy. This works like a charm: https://github.com/xtool-org/xtool

You do not need to upload to TestFlight or the App Store; you can just install the app locally to your iPhone via usb -- even from Linux!

When in doubt, just ask your coding agent of choice to help you create and upload a Hello World iOS app. It's really easy.