Apple

I watched a video recently which also said that reasons for Android losing ground to Apple is that the products have become more like each other. Android used to be seen as the cheap alternative to the expensive iPhone. But nowadays flagship Androids are just as expensive, if not moreso, than flagship iPhones. Apple also offer a much cheaper version of the iPhone which is comparable in price to low-to-mid-level Androids. Not to mention the fact that the longer OS support of Apple means that buying a cheap second-hand iPhone is easier and easier.

Couple that with the fact that, as you say, Androids are becoming less customisable while iPhones are becoming more customisable, and all the reasons that people used to choose Android are becoming less and less true and/or relevant.