No, it's only off the Play Store. But you can still download and install it from elsewhere. iOS is unique in that you can only install apps through the store; there is no side-loading.

Sideloading by developer access and by jailbreaking are things.

Does the former require paying $99/yr to Apple? If so no thanks.

And as for jailbreaking, the last time I jailbroke an iOS device was many years ago, and it never worked 100%. Every time I rebooted the damn thing it had to be connected to a computer to re-unlock it again. I don't know how the experience is now but it was terrible then. I haven't bought an Apple device since because I like the out-of-the-box ability to install whatever random stuff I want.

You don’t need to pay anything to develop on your own devices.

OK so can I use this to install Fortnite? How? And does it mean I need a Mac computer in addition to my iPhone?