where can I find a list of changes with each OS for iMac

I want to know what new stuff I have to learn.

The changes are mostly additional features, not things that are strictly necessary to learn.

macOS Monterey 12 (Fall 2021) was followed by macOS Ventura 13 (Fall 2022), then followed by macOS Sonoma 14 (Fall 2023), and finally the current version macOS Sequoia 15 (Fall 2024), and in the near future macOS Tahoe 26. Depending on the exact model iMac that you have, the maximum supported OS version may be less than the current one. Depending on hardware specs, some features may not be available to all supported models.

Identify your iMac model - Apple Support

Update macOS on Mac - Apple Support

Apple’s press releases give information on what was new in each major version.

macOS Ventura is now available - Apple Newsroom

macOS Sonoma is available today - Apple

macOS Sequoia is available today - Apple

The Wikipedia articles also detail what was newly introduced with each version.

New features - macOS Ventura - Wikipedia

New features - macOS Sonoma - Wikipedia

New features - macOS Sequoia - Wikipedia

At Ventura, the most jarring change I ran into was the change from System Preferences to (iPhone-like) Settings, accompanied by a complete shuffling of the organization of those sub-items:

System Preferences is renamed to System Settings and gets a brand new tabbed interface and re-organized panes based on the iOS/iPadOS Settings app.[5]

Because of this, the app-specific "Preferences..." menu bar item has been renamed "Settings..." for all apps.[5]

But it's not something you can study for -- you just have to use it.

There is one broad hint-- if you can't quickly find a pane you need, you can enter it in the search bar.

<< I want to know what new stuff I have to learn. >>

On a gross level, you have to learn nothing more than what you already know..

Each incremental version is intended to be backward-compatible, so you should be able to operate under the new version readily.

But if you want the extensive list of changes, Urquhart1244 has provided some great reference material.

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As always, Best Practice before a major upgrade is to make certain your local, disk-based Backup is completely up-to-date. That is very important because the only way to UNDO un upgrade is to ERASE your drive and all its files, and restore from that backup.