iOS is a very restricted operating system that doesn’t simply allow installation of 3rd party Apps, unlike Android which allows users to sideload any app without any complications. But if you wish to sideload 3rd party apps on your iPhone and iPad than there are few apps and methods to use any 3rd party IPA apps on your Apple Mobile.
Here are Steps to Install 3rd Party Apps on your iPhone and iPad with the help of the Alternative App Store.
Prerequisites for using iMazing to install Fused iOS apps directly to a mobile device
Conclusion: With the Steps above you can download and install 3rd party IPA files on your iPhone and iPad and enjoy tweaked apps and games, not on App Store.