Again, I've never seen a Patch named "Custom" in GB so I can't comment on that. When you Save your Preset , the Library pane just switches to show you the User Preset folder [in the bottom pane] instead of the usual Drum Kit folder. Your Patch is Saved into ~[your user folder] > Music > Audio Music Apps > Patches > Instrument > GarageBand. If you want to get rid of it, just navigate to that location in the Finder-- you'll see the Patch there, and you can send it to the Trash..

When I open the drummer, the region appears. But it has limited length. If I want the drummer to all the song, should I grab that region to the longer length till the song ends (that I will have one long drummer region in one song), or what to do? Is there sound quality difference between one long region (four minutes long, for example) or multiple regions of one track in one song? If multiple, is there usually a common number of regions (of one track) for one song? Thanks!

You can stretch a Drummer Region out or you can create multiple Drummer Regions with different tweaks to the pattern[s]. Keep in mind that, depending on the settings, Drummer automatically puts fills in, so you *will* get different results with one longer Region than with several shorter Regions. Once again, there are no rules -- you'll have to experiment yourself to see what you get with particular Drummers, and what works best for you in particular songs.

You say, drummer automatically puts fills in, I don't understand that sentence. What it means? What are fills? Are these fills the same fills which are on the controls? I also don't know, what the controls fills mean or do.

Fills are the little figures that drummers play occasionally or when they transition from one section to the other -- they interrupt the steady pattern to play a figure, or fill. Turn the dial toward Complex in the gird to hear more fills, and you'll get the idea.

Yes, I used the fills and they are like additional sounds. Thank you!

About user preset, maybe a bug, but maybe a Mojave's bug.. ?

Maybe a bug, but I'm on Mojave and I've never seen a "Custom" preset, so more likely a GB version bug [if it is a bug]..