On iPhones, Apple forbids third party appstores. You cannot also sideload. So that's a no go.

On Androids, I think you can install third party appstores. fdroid is the one I've heard before, but there could be more.

In the EU, iPhone users can install from alternative sources:

https://support.apple.com/en-gb/117767

It's still not like Android though.

> All apps available through alternative app distribution are submitted to a Notarisation process with Apple.

>On iPhones, Apple forbids third party appstores. You cannot also sideload

Very sad state of affair considering we are progressing into the 2nd quarter of the 21st Century, and some people are not complaining but willingly embracing (paying) into this by design incapability in the name of security and convenience (read profit margins).

It's methaphorically like willingly living in a country with virtual great wall around with a benevelont dictator but it's one of the life choices that people voluntarily need to make nonetheless.

Interestingly, moving countries (to the EU) could get you the option to break out of Apple's great wall.