I created an one note on piano roll, but got three of them. Why? See also a region on timeline. And why is the region rugged one, not plain as normal regions? Video is included. https://youtu.be/Htq48prEt-4

You see three notes there because you've Looped the Region -- those indentations indicate repeats of the Region. If you drag on the lower right corner you change the length of the Region, but if you drag on the upper right corner you'll cause the Region to Loop (repeat) -- that's apparently what you did. You can drag it backwards from the upper right to get rid of any unwanted loops/repeats.

But if I have looped the region, does it mean how chorus will be made? How do you make chorus (repeating notes) ?

Looping repeats a Region for as long as you drag the right edge of the Region out (to the right) from the upper right corner. I don't know what that would have to do with a chorus, or what a chorus would have to do with repeating notes..? If you want to have a Region repeat you can Loop it for as long as you want, or if you want to have notes repeat within a Region you can play/enter them in again or copy and paste them..

What is the difference between midi keyboard and piano roll? I mean, in placing notes? I understand on a midi keyboard is more easy to do that.. ? Anything else.. ?

MIDI notes are the same whether they're entered by penciling them in or playing them in live in realtime from a MIDI keyboard, but notes played in may have different Velocities, for a more musically dynamic performance than notes penciled in, which will all (initially) be at the same Velocity.

I penciled a note. Then I wanted to get from it multiple notes. I highlighted a note and hit alt dragged it far away from source note, but only one new note I got. Can I copy from one note multiple notes? How?

Not by Option (Alt)-dragging a single note. If you select multiple notes and Option-drag one of them you'll copy all of them. Or you could select one note, Copy it, and repeatedly locate the playback cursor to the desired location and Paste. But to do what I think you're trying to do, you would drag the Region from the Loop icon in the upper right corner, and the content of that Region will repeat for as long as you want (you'll get as many copies of the content of that Region as you want depending on how far to the right you drag, whether the Region contains a single note or multiple notes). Check out video #20 (in the course attached at the top of this thread) starting at around 1:20, to see this..

Hi. Thank you for the video, I checked it and already know how to loop the region. Also I know option dragging. But you have third suggestion (your sentence) : Or you could select one note, copy it, and repeatedly locate the playback cursor to the desired location and paste. Sorry, I have no idea, how to do all of that. Do you have any videos or screenshots... ? Thank you very much!

? What that means is just Copy & Paste using the standard method of copying & pasting that works in all applications: select; Copy (Command-C); locate; Paste (Command-V).