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I am thinking about buying myself a mac. The problem is that I don't want to use my Mac with my AppleID because I am very afraid that it will sync with my other apple devices and I'm not a big fan of this at all. I also don't want to maintain a seperate AppleID. Do I really need an AppleID for macOS and which parts of macOS I cannot use without an AppleID ?

No, you do not absolutely need one.

It is mostly used for synchronizing with your other devices, which you’ve stated that you do not want anyways.

However it will still be most practical if you create a new Apple-ID to be used exclusively for that computer. If you do not, you’ll not be able to download software from the App Store.

Note that you do not have to enter that Apple-ID in any of the systems that would otherwise sync data. Your files, photos, calendars, mails, contacts, etc. will stay completely private and not synced with anything, even if you use an Apple-ID for purchasing or otherwise downloading software from Apple.

The problem is that I don't want to use my Mac with my AppleID because I am very afraid that it will sync with my other apple devices and I'm not a big fan of this at all.

You can turn off sync and iCloud which will stop devices syncing.

Do I really need an AppleID for macOS and which parts of macOS I cannot use without an AppleID

An Apple ID is heavily recommended. A lot of services will be disabled. You won’t be able to download any apps from the App Store or iTunes.

These are some of the most widely used services that you access with
your Apple ID:

• App Store




• Apple Music




• Apple Online Store




• Apple Retail services and programs (Concierge, Joint Venture, workshops, and youth programs)




• Apple Store app




• Apple Support Communities




• FaceTime




• Find My Friends




• Find My iPhone




• Game Center




• Books Store




• iCloud




• iMessage




• iTunes Genius




• iTunes Home Sharing




• iTunes Match




• iTunes Store




• iTunes U




• Mac App Store




• Photo Print Products

No you don't need one at all. I don't get what with those recommendations. Just like Windows 10, you will not able to sync to iCloud (equivalent of OneDrive), download apps from the store, sync contacts, photos and pretty much all apple services that needs an apple id.

You can still perform updates, sideload apps with pkg or app extension.

Thats about it.

It would seem to me that you could setup the new Mac using an account tied to your Apple ID. You can set this account up to not sync with your other machines. You could also create another account which does not have an Apple Id. You could then choose which account to login to.

I assume if you sign out of iCloud before logging out, the the account will not sync. You could then always log in to the account without an Apple Id. You would only log in to the account with the Apple Id when you want to use the Apps Store.

A more drastic approach would be to actually delete the account with the Apple Id after downloading from the Apps Store.

I have a Mac that is not associated with an Apple ID. There are a few caveats such as being unable to obtain software from The App Store and content from services listed in Daniil Manokhin's answer.

Software can generally be found from other sources such as vendors' websites with the exception of new operating system versions. These can be copied from another Mac that is associated with an account or they can sometimes be found on the internet from sources of varying repute.