Bands 1 and 8 are Filters -- that means they do not boost, they only cut [remove, not move]. Specifically, a Filter cuts all frequencies beyond the Cutoff Frequency [the chosen frequency]. Band 1 is a low-cut [also called a High-Pass] Filter -- it cuts [removes] all frequencies below the Cutoff [chosen] frequency and passes all frequencies above the Cutoff [chosen] frequency; band 8 is a high-cut [also called a Low-Pass] filter -- it cuts [removes] all frequencies above the Cutoff [chosen] frequency and passes all frequencies below the Cutoff [chosen] frequency. A Filter more aggressively removes frequencies beyond the [chosen] Cutoff Frequency then a shelving EQ set to cut, though with some signals [and with some settings] the audible difference may be subtle. A typical example of when to use a filter would be to employ a low-cut [high-pass] filter to remove unwanted low-frequency thumps [footsteps] or rumble from a vocal recording.

I don't understand. I watched the video and there is shown that low pass filter removes 5k-20 only? and high pass filter removes 20-100 only? Right? What about 100-5k, the central range? No filters work there.. ? https://youtu.be/7IyqS1roQx0

GarageBand's EQ includes a lowpass filter and a highpass filter. You can set the Cutoff Frequency for each filter -- the highpass filter [band 1] will remove frequencies below the chosen Cutoff, and the lowpass filter [band 8] will remove frequencies above the chosen Cutoff. If you want to EQ frequencies in the midrange use bands 3-6.

But bands 3-6 are not filters. I understand that filters are removing a back noise, for example, but 3-6 only boost or cut. So if I can set the cutoff frequency for each filter, does it mean I can use filter in 3-6 area? Or what it means to set the cutoff frequency for each filter and how to do (set I mean) it?

The Cutoff Frequency of a filter is the frequency [that you set] that determines where the filtering will begin [above or below the Cutoff, depending on whether it's a lowpass or highpass filter]. You can change it be dragging horizontally, the same way you change the Center Frequencies of the other bands. If you change the default Cutoff Frequency of one of the filters in GB so that it falls within the same range as the default Center Frequencies of the other EQ bands, then yes, you can apply filtering within the midrange. An example of this might be to filter out low frequencies below 300 Hz and filter out high frequencies above 3 kHz on a vocal part, which produces a special effect -- a very midrangy vocal sound known as the 'telephone effect'. But other than special effects like that, most of the time highpass & lowpass filters are used to remove just the highest and lowest [unwanted] frequencies, when necessary.

OK, I think, I got it. Now, what is this and when I need to use it? https://youtu.be/Nk7MpHu8ULU

That level control on the right is just a final overall output level that affects the entire signal. If you apply a lot of EQ, the overall level of that signal might sound noticeably quieter or louder to you than before you EQ'd it, changing the balance of that track in the mix -- you could set the EQ'd signal to have the same level as the un-EQ'd signal with that overall level control [you'd set the level by ear]. If you're not applying so much EQ that the overall level of the track seems to be noticeably different, then there's no need to adjust that control.

What numbers are just on the left side of the final overall output level?

The numbers next to the overall output level slider are a dB scale for the level slider [dB is a measurement of level/gain]. That scale shows how much [by how many dB] you're raising or lowering the overall level.

Well, now we have been talking about the "regular" eq, but there is another one, like eq plugin. I can't find it now to send a video but I remember I saw it anywhere. Does it work like normal one? Or do I need to use that eq plugin at all? Do they have the difference in quality and in results?