What's the difference between AppleID and MacID?

What you're calling a "Mac ID" sounds like a local user account ID.

As Mr. Bennet-Alder explained, when you first set up your Mac, it creates a local account for you. This account is an Admin account, meaning that you can do certain things that require elevated privileges by supplying your local user name and password.

macOS lets you have multiple local accounts, if you choose, and the Admin users can decide which new users will get Admin powers. For instance, you could have

• The main account for yourself – with Admin powers
• An account for your spouse – with Admin powers – which would keep her files separate from yours, but would also give her the ability to change System Settings or install software
• Accounts for each of your kids – which lacked Admin powers – giving you a little more control over what your kids might do to your machine

It's not necessary to do things this way – but the capability is there, for those who want it.

Your Apple-ID account is used to make purchases and download software from the various Apple online stores, such as the Mac App Store, the iPhone App Store and similar.it is also used for iCloud mail, if you choose to participate. Your Apple-ID tracks all the hardware that logs in with that Apple-ID, and is required when updating or upgrading software. Since it is an online account, an Internet connection is required to validate your credentials.

When you first take your Mac out of the box and turn on your Mac for the first time, you create a User-ID and password. These are stored, encrypted, locally on your Mac and are not shared online in any form. The User-ID and password you enter is required when you start up your Mac. (If you are the only User, you may have chosen to auto-login, but that merely stores the login User-ID and password and enters them automatically at startup.)

This User-ID and password are the credentials required to grant read access to your files.

to expand on what Servant of Cats wrote above, MacOS is a multiple-User system. There is no "owner" account.

If you choose to create only one Account, the system continues to behave as if there were MANY Accounts, and works to protect each account from every other Account, particularly when it comes to file access. Admin Accounts merely have somewhat more abilities and somewhat more file access.