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.