Do the default bandwidth is wide (large) enough, that I can boost or cut a whatever frequency I wish to select? And what that bandwidth means? Upper mids are one bandwidth, mids are another one, etc, or how? Or does bandwidth is measurable with numbers which are on the both sides of eq 0-60 left and 30-30 right?

Bandwidth is the range of frequencies that's affected when you boost or cut a particular band. Each band has its own bandwidth, which you set with the Q control for that band. You can see how wide the bandwidth for a particular band is in the graphic -- it's the range of frequencies around the center frequency [dot]. So a wider bandwidth will have a more noticeable effect than a narrower bandwidth -- the effect of changing bandwidth may be more subtle than changing the frequency or the amount or boost or cut, but you should be able to hear the difference when you apply a boost or cut and adjust the bandwidth between wide and narrow. The scales on the left or right of the graph are not bandwidth, they're just the overall range of boost or cut.

OK. So bandwidth is something limited on the curve (you call it central line?). I tried a purple dot, dragged Q but it stopped on the number 2. Does any Q have his limits? I am not sure I saw something on the curve, do you see the change? Then I tried a green dot. You see what larger the number comes, in the beginning the colour on the line (Q?) goes wider but what larger number than more narrow the bandwidth on the curve comes, while finally only dot remains. And then I reduced Q (numbers) and on the line green colur appeared more, widely... Where is the sense of that? What larger number than wider Q has to be, right? If I choose (use) a Q, what I will notice - the number, or what happens on the curve? Does central line is the right word, or no matter which, in English? https://youtu.be/reoPwKo8kHo

In the previous post I explained that bandwidth [Q] is the range of frequencies that's affected *when you boost or cut a particular band*. You did not apply any boost or cut, so of course you can't see or hear any effect when adjusting Q if there's no boost or cut [or not enough boost or cut for the difference to be clearly visible]. Try applying a boost or cut of at least 6 dB or more [for example Gain: +6.0 dB or Gain: -6.0 dB] to a band, and then adjust Q for that band, and you'll see the difference it makes. Re your other question -- the frequency represented by the dot is called the Center Frequency for that band, not the central line.

Well, I boost and you see the Q is wider when the number goes smaller. Why is that? https://youtu.be/G6zioJShSis Another video, I have the sound. Do I hear sound goes more wider or has more volume when the Q on the line goes more wider? Do you listen to the same thing? But I boosted, in the beginning it arised but then dot moves itself to down as I was cutting it, but I didn't. I don't have a video of this as then I didn't record yet. https://youtu.be/WrZc9nL2_cg

Yes, smaller Q values are wider bandwidths, and larger Q values are narrower bandwidth -- it has to do with the technical way the Q value indicates bandwidth, but there's no need to worry about the math. You're not hearing a difference because you're cutting a frequency range where there is little or no signal from the piano -- notice that the spectrum analyzer display that you turned on shows which frequencies the piano sound is made up of, and it shows that the piano sound is mostly above the frequency you chose to cut, so there's nothing there to be affected by the EQ. Also the narrow Q values you're using would have a very subtle effect on tone anyway, and a narrow cut is much more subtle than a boost. If you're just experimenting with the EQ controls to see what effect they can have, try large boosts at various frequencies in the middle of the range -- any tonal variations introduced by the controls should be more obvious then..

I understand amp designer is related to EQ, right? Is it for real instruments' guitars or also can use with guitars of software instruments? Is it only for guitars?

Amp Designer simulates the effect of running a signal through a guitar amp, adding desirable distortion and providing broad tone controls. It's meant to be used with either a real guitar or a [clean] virtual guitar Instrument. Amp Designer is mainly meant for guitars -- like a real guitar amp it does have limited bandwidth so it might not be appropriate for many other instruments, but you could always try it on anything as a special effect.

If amp designer gives special effects for guitar, is it meant to heavy or rock music then? Not to the other styles?

Electric guitar is used in all kinds of musical styles, from rock to metal to pop to r&b to jazz. Naturally some of the Amp Designer presets will be suited for certain musical styles -- for example, heavily distorted settings would be appropriate to hard rock or metal -- but the different amps and the range of controls in Amp Designer can adapt it for the type of guitar sound that would be used in any musical style, from clean jazz guitar sounds to slightly crunchy pop/rock sounds to distorted hard rock sounds. Of course you'd have to have an idea of what type of guitar parts and sounds are used in whatever musical style you're going for in your own projects -- obviously the way to do this is to listen to a lot of songs in those styles you're interested in and focus on the guitar parts and guitar sounds, and then work with the settings in Amp Designer to get those types of sounds. In Amp Designer you could start with presets in the Clean category.. Load a preset, turn the Gain knob [on the left of the virtual amp front panel] to 5, and then experiment with it set at lower values [cleaner] and higher values [more distorted] -- some amps will create a lot of distortion while other will stay mostly clean; experiment with the three EQ knobs for darker or brighter tone. At that point it's all up to you, depending on the sounds you want for your music..