Audio units. Are they necessary to use them for a quality song? Most of the famous songs, do they have audio units, or not?

Most mixers apply processing to their mixes to improve the sound and add creative effects. Whether you utilize plug-ins (Audio Units) for processing, or how much and what kind of processing you apply, is up to you, based on what your ears tell you the mix needs -- it shouldn't really matter what other people do, every mix is different..

Do I need to install audio units or have I already them? Where I can find audio units exactly and how can I listen to all of them, to decide need I them in my mixing, or not?

All the plug-ins you see listed in the Smart Controls panel's Plug-in lists are Audio Unit [AU] plug-ins-- that's just the plug-in format that GarageBand uses [some other DAWs might use other plug-in formats, but GB only uses Audio Units]. The plug-ins you see in those plug-in menus are already installed -- they come with GB. You could buy additional third-party AU plug-ins, but until you've mastered the use of the built-in ones, you wouldn't be able to make informed choices. As to how you could learn about plug-ins.. Well, plug-in processing is a big part of how a mix is put together, and people study that and practice mixing with plug-in processing for years to learn about it. Ultimately the only way to really master using the various kinds of processors is to use them in context of a mix, and practice with many, many songs/mixes. That said, the course linked below discusses the various types of processors used in mixing. However that course is not conducted in GarageBand, and the plug-ins shown will usually be more advanced versions of the ones you might see in GB -- it's about plug-ins and processing in general, not specifically the ones in GB. But it does discuss most of the kinds of processes used in mixing. When you're ready to start mixing a full song with a lot of instrument/vocal tracks, you might find it a useful introduction to those tools as you begin to try to apply them in a real mix.. https://www.macprovideo.com/course/audiopedia-mixing-mastering

You write that plugins are audio processors. Does it mean, with plugins I can only process real instruments and a vocal? Not software instruments of garageband?

No, plug-ins can be applied to any type of track. Though the audio in Audio tracks comes from recorded audio (wav) files, and the audio in Software Instrument tracks comes from the virtual instrument [plug-in]s, it's all still audio.

So if I use plugins in software instrument, the track will be audio track then.. ? (blue on line)

No, an Audio track will still be an Audio track, and a Software Instrument track will still be a Software Instrument track. But of course they both always put out sound [audio], regardless of whether you do any processing with plug-ins.

So there are audio sound in an software instrument track processed by plugins and just a software instrument sound? Is there a difference in quality in both of sounds.. ?

I think this is getting lost in translation.. Audio IS sound -- audio is just another word for sound. Plug-in processors usually change the sound in some way, so yes, when you use plug-in processing on a track -- any kind of track -- it will change the sound, depending on what kind of processor it is. That's the point of using processing -- to change/improve the sound of a track.