Using macOS without an Apple ID? 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.