OK, interesting. I don't own an Android device, but I presume the type of apps that would make it into the App Store on iOS are all in the Play Store? So you wouldn't need to look any further for any mainstream (e.g. banking) app?
Installing the Amazon App Store appears to just be a one time act of side-loading (saying Yes at the warning). This may give someone pause who's just started with Android, but seems painless enough. Once installed, it just acts like the Play or OEM Stores?
I own and use both.
Things like banking and most financial (except for things like Samsung Pay) all come from the Play Store. At least I have never loaded any of those from elsewhere.
For all others it is pretty much a one time install/approval task and once installed it is just like any other store. As an example I have Play Store / Samsung / F-Droid / Aurora / Amazon on my Samsung device. Samsung for Sammy specific apps, F-Droid/Aurora/Amazon for apps that the Play Store doesn’t carry for various reasons.
Updating is simple and usually auto if I want. One thing I like is I can approve by app and/or site for non-Play Store items.
One last thing, I have apps that are on the Play Store and on an Alt Store - most Alt Stores it is easier to search - and pretty much all of them I can update via Play or Alt. Android recommends the Play Store.
Personally I do not understand the purported reality behind the “OMG it is an ALT STORE” angst.