Most App Store developers have opted out of letting you run their iPadOS or iOS apps on macOS. Most of the interesting stuff is blocked and the stuff that does remain is filled with weird UX inconsistencies. From my own decade of App Store purchases, I've found none that opted in to it that I thought were worth using on macOS consistently. It's not really a headlining feature and it still doesn't make for a productive software developer tablet experience.
Not quite. Apple requires app developers to opt into Mac distribution. The non-zero cost of doing so on top of testing costs for a new platform (testers may have iPads but not Macs) in addition to fear of the new ability to access the sandbox directly (the horror if players were allowed to edit-in 100,000 gems instead of buying them with IAP) killed a large amount of potential Mac support.
I mean, with Big Sur it's pretty hard to not see where they're coming from: big round borders, bloated quick settings menu and widgets that somehow manage to be equally as useless as they are on iPad... as a fan of 201x-2018-era MacOS, I'm pretty dissatisfied with the latest releases. Compared to Mojave, it feels much less like a professional workstation OS and more like a LeapPad with POSIX compliance.
Those are mostly design changes though, not a full blown merge of two operating systems. One could say MacOS looks closer to iPadOS every release, but that doesn't indicate they're merging.
The case could be made for either. On a technical level, Apple is depreciating their desktop APIs in favor of their mobile-first solutions (like Metal) as well as encouraging developers to target iPad/iOS first, then port to desktop later (with Catalyst). Hell, they even switched Mac hardware to closer resemble iDevice internals with their Apple Silicon switch. You can't really blame them, since mobile is their moneymaker and it's only natural for them to want to cater to them first, but it's hard to deny that MacOS is converging with mobile workflows and not the other way around.
