So cmd c, locate and cmd v? Repeatedly locate the playback cursor to desired location means again cmd c locate and cmd v? Or how?

Yes, to Copy & Paste a note select the note, Command-C, locate to where you want to place the note, Command-V. You should be able to do the same thing by Option-dragging the note repeatedly.

I watched the 6th video in 109 music theory for daw musicians. He divides the notes (scissors are seen), but he doesn't say, how he does it. Also, he makes two smaller notes as one bigger note, again not telling how he does it. I only saw a mouse on the note, scissors and voila! it happens, but how did it happen... ?

Remember, he's not teaching you how the DAW works in that course, he's just teaching you music theory. His examples are in Logic, so he's using Logic's Scissor Tool to divide notes and Logic's Glue Tool to combine two shorter notes (of the same pitch) into one longer note. Those tools are NOT available in GarageBand. In GB you can divide a note by selecting it, positioning the playback cursor where you want to cut the note, and hitting Command-T. Sometimes Command-J seems to work to join two shorter notes (of the same pitch) into one longer note, but most of the time Command-J DOESN'T work, so if you have two shorter notes (of the same pitch) you'd have to manually Trim the edge of one of the notes (and delete the other one) to make one longer note.

Master echo and master reverb, is it necessary to use them? Any particular music type? What is the difference of them (my dictionary gives for echo and reverb the same translation) ?

Echo is a discrete repeat -- it can be one discrete repeat, or multiple discrete repeats that tail off gradually; Reverb is thousands of repeats [echoes] all blended together -- you won't heat any discrete echoes, they're all blended together in a thick wash, like the sound in a large empty room. It is your choice whether you want to add reverb or echo to tracks in a song. Listen to music of the same genre as the music you make to get an idea of how they use reverb or echo in the mixes, and then you can decide for yourself if you want to use those effects. Btw, this thread is named Piano Roll, but reverb/echo is not a Piano Roll topic, they're effects, found in the Smart Controls panel -- please always start a new thread for a new topic..

You see, on the video I have looped a region. Then I decided that I want longer notes. I tried to change note on the region on timeline, but I couldn't. Then I changed the length of a note on the piano roll. I wanted a long note, good to listen to eq changes. But the looped region has the same short notes. How is that? How can I change the length of a note on the region? I understand the timeline' region and notes in piano roll are related to each other.. ? Or not then? Video is enclosed. https://youtu.be/G4fSWje2WOA

To make individual notes longer you would have to edit the MIDI notes themselves in the Piano Roll [as you apparently did]. Changing the length of the Regions above will not affect the lengths of the individual notes inside those Regions. But it doesn't matter how long a MIDI note is, if the instrument on that track plays only short notes, then it doesn't matter how long the MIDI note is, the actual audio note that you hear will stop when it's over. An electric piano is like that -- it doesn't matter how long you hold the note down, the actual audio note will play for a second or two and then fade out, even if you're still holding down the key [or even if you extend the note length in the Piano Roll, as you did]. Only instruments that play sustaining notes -- notes that can last as long as you hold down the key, or for the full length of the MIDI note -- will behave the way you're expecting. An example of that would be, say, an organ, or strings [but NOT pianos, or bass, or guitars] -- try different instruments and when you find one that can play longer notes and hold those notes indefinitely, then you can play or edit the MIDI notes [as you did] to make the notes last as long as you want.

I chose organ track and the length of default note is long, normally default is short (1/4?). Why is that? https://youtu.be/GZBTByfwfmg

The default note length is based on the length of the last note you select in that same Region, so if you select a short note [or edit a note length to make it short] and then pencil in more notes they'll be short, if you select a long note [or edit a note length to make it long] and then pencil in more notes they'll be long.