Install IPA on iPhone would be using online web service Install On Air. You can simply upload your IPA file for iOS to Installonair website and it will generate a link. When you open the link on your iOS device web browser, it will ask you if you want to install the app. Then you just tap on it to confirm the installation.
If you have the IPA file downloaded on your iPhone, you can also complete this process on your iPhone. When you tap on the Choose a File area, it allows you to upload the IPA from Files App.
If your IPA file is not saved in the Files app, you can tap on the Share button and select Save to Files. That is to say, you can install the IPA file you received from anywhere on your iOS device. If you have connected other cloud drive to the Files app, you can also install the .ipa file on iPhone from Google Drive, OneDrive, and another corresponding cloud drive.