Once you have the IPA file, you cannot simply open it on your iPhone. You need a sideloading tool:
However, this process sits in a legal and technical grey area. This article will cover everything you need to know: why you want an old version, the risks involved, where to find safe IPA files, and how to install them without a computer (or with one). youtube ipa download old version
This is the most common method for modern users. You download an IPA file and "push" it to your device using a computer. Once you have the IPA file, you cannot