Honeycomb, Android 3.0, was released Feb 2011 for tablets. It was a radical new UI and introduced several new APIs. It was the largest Android update ever, except that no devices were updated to it. Android tablets didn't sell very well and developers had little to no reason to support Honeycomb.
Ice Cream Sandwich, Android 4.0, October 2011. This refined the Honeycomb changes and was appropriate for use on phones. It did work on tablets but wasn't really optimized for them until 4.1/jellybean.
Honeycomb wasn't a fork or anything, but the strong divergence and poor sales of Android tablets meant that no one wanted to target Honeycomb until they were actually targeting Ice Cream Sandwich, and then at that point it was barely worth supporting Honeycomb tablets, but still work supporting Gingerbread phones.
I don't think it's particular relevant to the iPadOS situation.
Also relevant, that Honeycomb wasn't released into AOSP until ICS, which meant there were no third-party builds or even support from off-name brands.
Since most of the improvements ended up being useful for the upcoming "phablets" as much as tablets, Google just merged those features into Jelly Bean and gave up on a separate tablet UI.
No, not at all. Android's UI has scaled exceptionally well since long before then, and nobody on Android was using 320x240 screens at that time. Android had already long since done the high DPI jump from 320x480 (HTC G1) to 480x800 (Nexus One).
You need a UI that takes advantage of the extra screen real-estate, yes, but there was nothing about Android's core UI toolkit that failed to handle that in any way.
> Since most of the improvements ended up being useful for the upcoming "phablets" as much as tablets, Google just merged those features into Jelly Bean and gave up on a separate tablet UI.
Google never actually forked Android for tablets, so there was nothing to "merge." They just focused on the tablet UIs & devices such that the phone ones had bugs. You can boot up Honeycomb on a phone-sized device and poke about. Everything's basically there, it's just super buggy.