I've heard that Apple hardware in particular sticks with an AP longer than other devices, the Apple device is not continually looking for the best connection, it sticks with what it started with until the signal gets very weak. Don't take this literally but it's like Apple waits until you're down to one bar while some others wait until you're down to two bars before looking for a stronger connection from another AP.

You should make sure the APs don't interfere with each other. The automatic channel selection didn't work well for me. I had to manually set my two APs at each end of the 2.4GHz spectrum. It's also a good idea to turn on wide band channel. If you have an Android device somewhere you can download wifi analyser to make sure the channels don't overlap.

They don't need to be on different channels, but it's recommended for performance (due to co-channel interference).

Think of it this way: Each channel provides a fixed amount of bandwidth, and neighboring APs need to overlap slightly to provide seamless coverage. As a first order approximation, if two APs are on the same channel you'll have twice the range, but half the bandwidth because the spectrum would be shared between them.

(It's actually slightly worse than this, because collisions will happen causing additional overhead.)

I have a ubiquiti AP and as part of the set up it did a scan which picked my channels based on what it scanned and it seemed to do a decent job. You can go into the controller software to initiate a scan, though it will kick everything off the network for a few minutes.

If you option-click on the wifi symbol in the macOS menu bar, there's an option to access various diagnostic tools. One of them gives you channel recommendations as well.

Channel selection won't affect roaming, so you shouldn't need to. What specific problem are you having?