What is the main difference between Android and iOS?

Android is a free system that can be used by any phone maker.

Each tends to modify it to try and make it somehow special to "their" phone.

iOS is by Apple for their own phones; they create both the hardware and software so the overall system is well optimised.

In many ways they are very similar; iOS originates from BSD Unix, if I remember right, and Android is basically Google's extremely customised and modified version of Linux (= Linus's Unix), 

So, both descendants of Unix via convoluted paths. And both Android and iOS devices use ARM CPUs.

Apple "vet" apps more strictly than Google, so rather less, but less trivial or badly written ones in the apple store than in the android one.

With similar price & spec phones, like the top Samsung vs latest Apple, there is very little to choose from - it's personal preference which menu system and app store you prefer.

I use both android and ios devices for different things.

The only point that sticks in my mind is that Android phones or tablets never seem to have long term updates; the makers have to re-modify each new version of generic Android to re-apply their changes, which takes time and effort - so often they do not bother.

Apple issue updates as long as the phones are capable of running them - five years or so, from past experience.

Just about everything.