I watched audiopedia 7th video, but it doesn't help. As on your video, there are numbers below the curve (center frequency, gain and bandwidth), but in eq in my gb I don't have these numbers. All I have are bottom right central frequency and gain, but if I use gain it affects only above of it dot, not for example a yellow dot. And I don't have a symbol up left at all. https://youtu.be/K3OnFoNd_5E https://youtu.be/OhEtuzonB-Q

As I mentioned in a previous post, the EQ in the videos is not the GarageBand EQ, it's the Logic EQ -- the Logic EQ is the same as the GarageBand EQ except that the GB EQ only shows the numerical display for one band at a time [the currently selected band], while the Logic EQ shows them all. So when you select a particular band in the GB EQ the numerical display will change to the values for that particular band. For example, if you want to use the numerical display to change the values of the band that corresponds to the yellow dot, then you have to first select that band [click on the yellow dot], and then you can adjust that band via the numerical display [you'll notice that the color of the numerical display changes to indicate which band is currently selected].

Yes, it works, I click on the yellow dot. But now if I select Q, then it doesn't make a cone (wedge) like on your video. What Q means? A cone? Frequency button moves dot from left to right and gain moves up&down, right? And if I don't like to use them, I can move dot itself by mouse and that's the same? And you see, Q (a bandwidth?) is under purple. If I click on purple shaded area (not even on purple dot to select purple), the selected yellow band deactivates and purple comes instead of it. Why? https://youtu.be/5RzRSV3JN4w

"Q" is the bandwidth of the selected band -- how wide a range of frequencies will be affected when you boost or cut the selected band. Q is not specifically related to the last [purple] band, it's part of the three numerical controls for the currently-selected band -- the numerical display for Q just happens to be on the right side, under the default position of that purple dot. Clicking in the shaded area is the same as clicking on the dot for that band -- it selects that band. And if you don't want to drag on the numbers you can just drag the dot [or even just drag in the shaded area], it's the same thing [you'll see the numbers for the currently-selected band changing when you drag on a dot -- as you noted dragging left/right corresponds to frequency, dragging up/down corresponds to gain [boost/cut]]. In the video you're dragging the Q for the band that corresponds to the blue dot -- there's very little boost in that band so you can barely see what it's doing, but I can see that band getting getting wider, so it's working as it should. Then you must have let go of the mouse button when the cursor was hovering over the purple zone so you [accidentally?] selected the purple band. While you're playing with the EQ are you actually listening to the effect it's having on the tone of that track? -- in the video the transport is not moving.. Maybe it'll make more sense to you if you're hearing what it's doing as you're practicing adjusting it, after all that's the whole point of EQ -- there's no point in just playing with the controls visually, it's not about shapes and graphics but about the tonal changes it creates when those controls are adjusted..

There is a sentence: Boosting the guitar around 3000hz will help it cut through the mix, especially on lead lines. I don't understand. Boosting is dragging dot up and cutting is dragging dot down. So how boosting the guitar helps vice versa, cut the guitar.. ? And what means on the lead lines? Where are those lead lines, how they look like and are they related to guitar or eq?

"Cut through the mix" means making a track/instrument slightly more prominent, possibly helping it to be heard a little more clearly against other instruments in the mix. "Lead lines" means melodic phrases where an instrument takes the role of the main melody. Keep in mind, suggestions like that are only generalizations -- the specific EQ that a mixer may decide to apply will always depend on the particular recording, the particular mix, and the particular arrangement..

There are 8 bands above (symbols). If I don't want to use right bottom (numerical display) Q, then where is else Q exist?

In the GarageBand EQ, it appears that the numerical value for Q for the selected band [at the bottom] is the only way to adjust Q [bandwidth].

If I don't adjust Q, can't I boost or cut the frequencies? As you see, I still can move a dot up or down, right or left without Q selected.... So Q is not necessary?

Q adjusts the bandwidth of the selected frequency. If you don't want to change the default bandwidth then you don't have to adjust Q, you can just boost or cut at whatever frequency you select. It's up to you to determine [by ear] what settings you want to apply with the EQ.