The seller told me, that I need to do all updates and due it, the life-span of Mac will be about 7 years. Is it true? For example, I just updated Imovie, but I will never use it. Can I get rid of it, by deleting it anyhow.. ? Is it doable and is that good for Mac?

You do NOT need to do all OS and application upgrades, unless there are new features in some key application that you absolutely must have, and you absolutely have to upgrade the OS for the sake of compatibility. Many musicians and studios run on 2-3-year-old OS's -- "if it ain't broke, don't fix it". Traditionally you'd expect to be able to get around 4-6 years out of a Mac at least [I stuck with my old iMac from 2011 to 2019], but Apple has accelerated the rate of OS updates which may reduce compatibility with older models more quickly. With the introduction of new hardware [the new Apple-manufactured M1 CPU] that will complicate the equations -- I wouldn't go for one of those new machines for at least a couple of years, to wait for all the bugs to be worked out and all software/hardware/drivers to achieve full native compatibility. There's no reason to delete any of the applications that come with macOS -- I'd leave them there, but just don't use them if you don't need to.

Are you very sure, I don't need to do all updates? OK. Can I delete Imovies, Numbers etc from my Mac at all? No updates, no take memory.. How can I get rid of that number which shows me on the dock that new update is available? For example, on the dock I have right now system preferences and there up on the right corner number 1, which reminds me A Big Sur is coming.. I don't want to download it and either don't see that number also.

If the currently-installed versions of your computer OS and applications do everything you need then you don't have to update just because updates are available. I wouldn't delete the applications/utilities that are installed as part of the macOS, but if you really want to then go ahead. But why so worried about memory --are you running out of hard disk space? If you really need to clear space on your internal hard disk I'd suggest adding a large external hard disk and either keeping all your large audio Projects on that -- a standard approach for most professional studios -- or moving libraries like iPhotos, iTunes, and iMovie over to it.

Am I understanding rightly, that I can keep my gb projects also in Itunes, Iphotos and Imovie? What are they exactly? You call them libraries, does it mean, I can call them storage places, like Icloud? Is Icloud also a library?

If you're mostly unfamiliar with the terminology, organization, and [many of the] basic components of the OS it sounds like you'll probably want to read through the documentation for macOS and your particular model computer. These links are to the manual pages in Apple support where you should be able to get the appropriate Apple manuals for your OS version, Mac model, and included Apple software. Specify your Mac model and OS version: https://support.apple.com/guide/system-information/find-the-manual-for-your-mac-syspr35535/10.14/mac/10.15 https://support.apple.com/manuals/macnotebooks https://support.apple.com/manuals/macdesktops https://support.apple.com/manuals/consumersoftware https://support.apple.com/guide/icloud/welcome/icloud