Can You Install 3rd Party Apps On IPhone?
No. IPhone really care about users privacy so apple don't allow any third party application on an IPhone. It only support the apps designed for its OS only.
Can you install apps on iPhone without App Store?
For most people, that's absolutely true. The vast majority of apps on iPhones can be installed only through the App Store, and Apple doesn't offer an official way to install software outside of the App Store using an installation file downloaded from the internet, a process called “sideloading.”
Three ways to get iPhone software without using Apple's App Store
Does iOS 14 support 3rd party apps?
In iOS 14, Apple allows you to limit the level of access that third-party apps have to the photos library on your iPhone or iPad.
iOS 14: How to Limit How Much Access Third-Party Apps Have