Need guidance regarding passwords and IDs

The User ID and Password is an Account the User created How to create a new Apple ID

Manage and use your Apple ID

The Computer Password was created when the computer was First Setup when it was new. Generally, this is not the same password as the Apple ID password.

As mentioned, Keychain is very useful and convenient. If you ake Keychain an iCloud app, you can "sync" passwords across multiple Apple devices (using the same AppleID).

BTW, I also suggest setting up a second backup administrator account (use a hint to help you recall the password) to be used when all else fails.

Well done.

Of course, there are many advanced topics such using account ids for disk access, etc. Also, complications using handoff features across multiple devices (e.g. I couldn't get my spouse's iPad to work as an extended display until I realized that I was logged in to my own account with a separate AppleID).

Your user account on the Mac has an account name and password.

Your keychain stores passwords for other types of accounts, like email accounts, websites, software licenses, etc.

It also stores the certificates needed to securely connect to websites or other certificate-based logins.

With keychain, you only need to remember your user account password in order to unlock other accounts on the internet.

An AppleID is used to log into iCloud and to purchase apps and content from the Mac App Store and the iTunes Store.

A brief overview for you -

On a Mac there is always at least one user account. The first such user account got created when you installed your Mac the first time you turned it on. That first user account is always an admin account because it has administrative privileges on your Mac. In System Preferences > Users & Groups you can create more users; if you give them administrative privileges they are also admin users; if not they are regular users that do not have administrative privileges.

Every user account on a Mac has an account id that you use to log onto your Mac; generically it's called a login id and every user account on your Mac has one. And of course since your user account is an admin account you could call your "login id" an "admin id."

Your Apple ID is a different thing. It is independent of user id's on your Mac but you can associate it with one or more of the user ids on your Mac. Your Apple ID enables you to install macOS, updates and upgrades; sync photos, music and other info between your Mac and other Apple devices you may own (other Macs, iPhones, iPads, etc); use iCloud; purchase, download & install apps from the Mac App Store; purchase music, movies & TV shows from Apple; access Apple Communities and more.

The Keychain is a database on your Mac where the system stores your various id's, passwords and other system information. Normally you do not need to do anything with the Keychain and it's advisable not to go poking around in it unless you have a thorough understanding of the Keychain and how your Mac uses it.

Here's the Keychain User Guide:

What is Keychain Access on Mac? - Apple Support

Keychain Access User Guide for Mac - Apple Support

Thanks for the compliment! 👀

Of course, you asked for the "idiots guide to passwords and id's," so that's all I wrote! As you know, there's lots more to it! Good luck with everything!