Before TestFlight was a thing, you could self-sign your own apps (.ipa) and install them to local devices through iTunes over a USB cable connected to the device. The developer signing certificate for this was/is free, included when you sign up for the free version of Apple Developer account.

Nowadays it looks like you can still do this directly from Xcode. See section: “Connect real devices to your Mac”

https://developer.apple.com/documentation/xcode/running-your-app-in-simulator-or-on-a-device

*The mention of Apple Developer Program in the bullet points of this section is an “if” and is optional. It’s not required for testing apps on local devices.