There are ways to sideload apps on iOS, but it's pain in the ass. From my memory (may not be 100% correct as of today):
many 3rd party apps not allowed in apps store are distributed in source code form. use xcode to compile the app yourself and load it to your phone. Works for a week, then you'll have to install it again. Oh, and you can only install three at a time.
there are 3rd party apps stores that sort of working by distributing their developer certificates along with the 3rd party apps (e.g. tweakboxapp, etc). You'll need to trust the developer certificates before you can install the apps. Oh, and apple frequently revoked their certificates so you'll going to play cat and mouse with apple.
use altstore.io to install 3rd party app. The same 3 apps limit still apply, but this app automatically manage reinstallation for you, so it can reduce the pain of managing sideloaded apps just a bit.
Loading your own software on a device independently of an app store, a.k.a. "the way general-purpose computers are supposed to work," a.k.a. "exercising your property rights."
On iPhone you can only install an app via AppStore. Side loading is the act of installing an app without the AppStore (with a different store, or by sending a file from your computer… it allows to install apps that Apple doesn’t want on their store for various reasons like a retro game emulator to play Super Nintendo on your phone.
The really frustrating thing about iOS is even if you make and computer your own app, you have to reinstall it every week despite paying a $100 developer fee. And you can only have a few apps installed that way.
Yes. I use SideStore which allows you to renew the app right on your phone without a computer but it’s limited to 3 apps and one is SideStore so really only two sideloaded apps.