So how to use a metronome? I understand there is a metronome click. Should I play a note on midi keyboard in the same time as that click is coming? Or how?

The standard way of recording in a DAW is to turn the metronome on, choose an appropriate Tempo, and record in realtime from a MIDI keyboard, using the metronome click as a guide -- play in time with the metronome clicks, so the notes you play will be in time with the bar & beat ruler in the DAW, making musical editing easier (check out that same course I attached to the other threads in this series, video #6 on).. https://www.macprovideo.com/course/music-theory-109-music-theory-for-daw-musicians?afid=E470KLQ7r9

OK. So I understand that I have to play same time the note and metronome click. But metronome click is short always, but notes are different. I use different length notes, the metronome click is the same. If I use a full note, it passes many clicks... How the different notes fit together with metronome certain length clicks... ?

Of course you don't have to play a note for every metronome click, the clicks are only there as as guide to playing in time with the beat. You play notes whenever you want, and they can be as long or as short as you want them to be. They can start on the beats or in-between the beats. But the beats/clicks are a guide to playing in good musical time (consistent rhythm). Listen to some music (any pop music with a steady rhythm) and tap along with the beat as you listen -- if you tap in good time with the music the beat you'll be tapping will be the equivalent of a metronome click, but while the notes will be in time with that beat they'll all be different..

What happens if I don't use a metronome? I have read from google that there is a good quality pop-music created without using a metronome...

You don't have to use a metronome, that's perfectly fine. If you don't play along with a metronome click your recordings will not be in musical time with the bar & beat ruler, and you will NOT be able to - quickly & easily make musical edits on the beat - quantize MIDI or audio tracks - properly use Apple Loops It's your choice.

Hi.You wrote that if I don't use a metronome click I will be not able to quantize midi tracks. Is it the same as time quantizing? Because I have sent a video how time quantizing works (not in this topic) on my project. And it works without using a metronome click. The benefit of catalina.. ? Also you wrote then I will be not able to "quickly and easily make musical edits on the beat". What means musical edits on the beat? How many and which choices are there? What I can edit on the beat.. ?

Quantization works by moving notes to the nearest musical gridlines. The metronome clicks on the musical [quarter-note] gridlines, that's how you know if you're paying more or less in time with the musical grid. If you don't play along with the metronome, then how would you be able to play close enough in time to the musical grid for Quantization to work properly? Keep in mind that if you pencil in notes manually one at a time instead of playing them in in real-time then it doesn't matter -- the metronome is only needed for real-time playing/recording. https://www.macprovideo.com/course/music-theory-109-music-theory-for-daw-musicians?afid=E470KLQ7r9

You wrote that I have not to play a note for every metronome click. OK, that's I understand. How many notes I need to play for a metronome click, that the result shows the proper use of metronome (as I intended to) ? 50% of the notes.. ?

You asked > how many notes I need to play for a metronome click, that the result shows the proper use of metronome (as I intended to) ? I don't understand this question [maybe it's lost in translation?]. You play the notes you want to play, the rhythm or melody or chords or whatever. If you play along with a metronome click then if you decide that your musical timing was not as accurate -- as close to the musical grid -- as you intended, then you can quantize the start points of the notes to fall on the nearest gridline. You can quantize all notes or select some notes and quantize just those. What you quantize is determined by you, based on musical considerations.