Anything that requires access to Apple's service – TestFlight, push notifications, Game Center, etc will require payment. If all you want is to stick an app on your phone that's free (as of ~5-6 years ago?).

There are limitations. The signature on the binary is only valid for 7 days I believe, so naively you'd have to rebuild/reload every week, however there are certainly automations that do this for third party apps with a little daemon running on your Mac in the background, and I suspect there are equivalent automations for your own apps/open source codebases to reduce the hassle.