Drummer is more of an automatic pattern generator -- what you're really doing with the controls is selecting internally programmed variations of each particular beat. It's meant to be like a producer making suggestions to the drummer -- the drummer will adjust the performance in response, but ultimately it's the drummer -- in this case "Drummer" -- who decides exactly what to play. Drummer is not for those who want to precisely control the drum parts -- it never gives you that degree of specific control [especially when it comes to fills]. Those who are more particular about the specific details of the drum part [like me] may choose to use the Drummer kits, which are very good, but will probably want to program their own patterns and parts.

I don't understand. If you say, the drummer decides what to play, so I can choose a style like a kyle and a beat preset like a half-pipe, and that's it? Because I don't decide? And swing is optional.

No, that's not it -- as I said, you do have some options to affect what the "Drummer" plays, but you can't be that specific -- you can push the "performance" this way or that but the exact changes to the part are decided by Drummer's intelligent algorithms. If you want a better understanding of how Drummer works, I've changed the link above to a course that focuses on Drummer specifically -- the course looks at Drummer in Logic, but Drummer is pretty much the same in both Logic and GarageBand -- the first 12 videos cover the "performance" and programming aspects of Drummer.

Thank you for the course, it was helpful! If I choose a beat preset, then in the end of the list comes custom. If I choose another pick, that custom disappears to nowhere. If I choose file and save, it doesn't save the custom in case if I like it and not want the disappearance. What kind of custom is that, why it disappears and how can I save it? Video is included. https://youtu.be/xD6vIArdknU

I can't duplicate that -- I don't see any preset named Custom for that [or any other] Drummer. If I had to guess, maybe you tweaked the controls, and perhaps it was another reminder that if you wanted to keep those settings permanently you'd have to save them, which you would do via the Save button in the lower right corner of the Library panel.

You see, the video starts with Anders-hard rock, last beat preset there is a sticks and stones in yellow colour. Then I change the drummer to Max-punk rock, and last beat preset appears custom in yellow colour. It's not only Max - if I change drummer again, his beat presets come with last choice as custom. Then I play that custom and saving that file (with custom?), but the custom never saves...

I saw it in the video, but as I said I can't duplicate that behavior here -- no matter what I do in Drummer I never see a preset called Custom. If it doesn't save as expected, and the other presets do, then I'd recommend you simply not use it -- just use the other presets. Maybe it's a new feature that's not in my version, or a half-finished feature that shouldn't be showing up yet, or just a bug, but I can find no reference to it in the documentation and I've never seen it a GB video AFAICR..

OK. Now I have put together a random drummer track and two other tracks. But they don't fit very well. How to make things better? Using the groove track? Or trying to find better matching drummer one? But if none of them fits together with other tracks and I still want to use drums, do I then just need to select a drum track?

When you say "they don't fit very well", if you mean that the timing between your tracks is not as tight as it should be, then making the Drummer track the Groove track and setting the the other tracks to follow its timing *might* help. But beyond that I can't comment on the musical aspects of your song without hearing what you have -- and even then it's going to be your own musical judgement that makes the determinations of what works and what doesn't..

I hit save button in the lower right corner of the library and it gives me an opportunity to save (that custom?) as patches. Well I saved and now if I choose that drummer it shows him under patches, but nowhere anymore custom appears.. And not beat presets appearing. Instead of them is keyboard sensitivity. Other drummer choices as drummers (as Kyle), sounds are still exist. Well, I closed project without saving the file and now beat presets are back, but all the customs are still gone and that user patch is still exist. How can I get rid of (my selected) a user patch?