I try to do it. I choose limit to key and 100%. Then I listen to my vocal. I understand, the pitch key works so? Then I duplicate a vocal track and remove limit to key and 100%. It means, a duplicated track has not pitch correction? I don't listen to the difference. Actually, I am not sure, choosing limit to key with 100% changes the vocal at all. I'm doing anything wrong.. ?

It sounds like you're applying GB's pitch correction correctly, but keep in mind that how well that works will depend on the particular vocal part you're trying to correct -- if the pitchiness in the recording is too inconsistent or wavering the pitch correction may not be particularly effective. The pitch correction in GarageBand does not seem to be anywhere near as effective as the more sophisticated Flex Pitch feature in Logic, but that's not surprising for a free DAW..

Do you listen to the difference of 2 vocals? https://youtu.be/TybKEeH50BA

You do realize there's no sound in the video..? To record audio in a QTP video you'd select the internal mic as the sound source for the QTP recording. However, I don't know if you can record audio in QTP that way if the sound is coming through the internal computer speakers -- you might have to run the sound from GB through external speakers. I guess you'd have to try it to find out what works with your OS version and setup. To record sound internally directly from a DAW (like GB) in a QTP video you'd have to install other software.

Where can I select the internal mic as the sound source for the QTP recording?

In QTP in my OS version (Mojave) there's a little triangle next to the Start (Record) button, which opens a popup menu where you choose the source if you want sound in the video. It looks like in QTP in Catalina OS or higher that little menu is accessed from the button labelled Options (again, next to the Record button)..

Thank you! I chosed built-in microphone and got the voice, very easy, thanks again! Now I enclose a video, but I don't hear the difference - one of them has to be as pitch correction... https://youtu.be/VUTACsBytQg

I don't really hear a difference either, but the pitch is not steady so it's hard to say.. You didn't say which track is the pitch-corrected track, but other than the unsteady pitch neither track seems to be especially out of tune, so I wouldn't expect to hear much of a change from pitch correction anyway. Why are you pitch-correcting that track -- does it sound out of tune to you? What are you expecting to hear different? All that pitch correction is supposed to do is move the pitches of notes to the correct pitches -- if the notes are already on or very close to the correct pitches, there should be little or no change.

You ask, why I pitch correcting that track. Well, I don't hear is it out of tune or not, but I have practicing almost all things I have had questions about; now I'm practicing a pitch correction, to get know does it works on me as video shows. One thing is to see how anyone is doing something, another thing is to be capable to do it my own. So now I have reached to the pitch correction... But I don't hear the difference. It means, maybe I'm doing anything wrongly? I expect to hear more fitting voice and music. I don't believe I can sing in the right pitch correctly. Pitch-corrected track is the first one; another one is a duplicated track. Does pitch correction effect the region, or the track? Otherwise, I copy the region to duplicated track, but that region is already a pitch-corrected, I mean I copy the same, so how can I listen to difference anyway then.. ? Or if the pitch-corrected region copies to the new track and on this track I haven't used pitch correction already, it means that new, duplicated track "removes" a pitch correction from the region.. ? As when the region was in the first track there it is with pitch correction. If I move (copy) it to the duplicated track, the pitch correction disappears there (on the duplicated track) ? Right?

Ok, it's good to practice the techniques, but you can't really practice pitch correction with a track that is not out of tune -- there'll be nothing to correct, so of course you won't hear a difference. In GB pitch correction applies to the Track. Down in the Editor, if you apply something with the Track pane selected it applies to all Regions on that track; if you apply something with the Region pane selected it only applies to the currently selected Region.