To install a .ipa file on your iPhone, you can follow these steps:
Use iTunes
Connect your iPhone to your computer and open iTunes (if using macOS Catalina or later, you won't have iTunes).
Drag and drop the .ipa file into the "Apps" section of your connected iPhone in iTunes.
Click "Sync" to transfer the app to your iPhone.
Use Apple Configurator 2 (macOS):
Download and install Apple Configurator 2 from the Mac App Store.
Connect your iPhone to your Mac.
Open Apple Configurator 2 and click on the "Add" button.
Select the .ipa file and it will be installed on your iPhone.