Welcome!
I have some knowledge of both iOS and Android and would be happy to try and answer your questions, though as Amoun says, there’s quite a lot of info around the forum and doubtless elsewhere. A couple of suggestions
Are you also looking to avoid Google (the main mover of the Android world)? In that case we can also make suggestions regarding alternative editions of Android, but your best bet is probably to use the stock Fairphone version of Android to make the move from Apple, and then think about it once you’re used to Android.
Don’t hesitate to ask.
P.S. the “seamlessly” mentioned on android.com is doubtless a bit optimistic. Apple devices come with software from the same company, a fairly closed world. In Android, Google can provide services equivalent to iCloud (back-ups, mail, drive, sync …) if that’s the way you want to go, but in Android, other solutions exist.