What I'd add is:

On some Windows layouts (those with the right alt = ctrl+alt trick), right alt is literally mapped to ctrl+alt. Right alt cannot be used to open the menu bar like the left alt can. This is only the case on those keyboards, on other keyboards, the right alt does move focus to the menu bar.

Mac uses cmd+arrows like Windows uses home, end, ctrl+home, ctrl+end. It uses option+arrows to move by word instead of control. (I doubt that what it considers a "word" is identical to Windows either, though many editors do override this anyway.

On some layouts (Polish is one I know of, though there are probably more), shift+backtick is a dead key on Windows and can be used to enter diacritics. Shift+accent followed by c inserts "ć". This is very painful to C++ (and Elixir) programmers, doubly so if you use the cFooManager systems Hungarian notation for class names. This is not the case on the Mac.

Option+letters are semi-dead (zombie?) keys on some layouts. For example, option+u inserts the ¨ accent, but option+u followed by another u combines them, so it inserts an ü. If you follow such a zombie key by a space, the space is swallowed and only the accent is inserted. Polish stupidly copied Option assignments from the US layout, replaced some with Polish diacritics, but never gave us replacements for the removed accents. So, we can now type symbols as useless as Ī or Ŕ, but "el niño" or "résumé" require the key hold and pop-up.

MacOS relies on menu options for things like copy and paste, as well as "Quit" and "Close Window". You should implement them by using the storyboard xCode gives you by default (or meticulously re-creating its included menu bar programmatically if your toolkit of choice doesn't let you do that), and then handle the appropriate selectors in your app if you need to. You should never handle these keyboard shortcuts yourself. MacOS is usually smart enough to figure out when these menu items should be dimmed and when they shouldn't. This doesn't work for custom UI frameworks, but if you use those, you ask for that kind of pain.

On both platforms, IMEs exist. Speakers of Chinese and Japanese want to type too.. Let them type. This involves painful OS integration that a lot of frameworks aren't doing.

On Mac, Option+backspace deletes to the beginning of the current word, but it keeps the space. On Windows (I think???) ctrl+backspace deletes the space.

On Windows, the shortcut to delete something (as in, an item in a list) is the delete key. Shift+delete is "irreversible delete" / "erase", AKA bypassing the recycle bin. On Mac, that's Cmd+Delete (which would be CMd+Backspace from a PC user's perspective).

On Mac, function keys are disabled by default (and perform the work of media keys). You need to combine them with fn to get the original behavior. Some older Macs (including older M-series macs) don't even have function keys at all, they instead have a touch bar. Don't rely on function keys for complex shortcuts.

On Mac, ctrl+arrows and command+space, while not being "reserved" in the traditional sense of the word, are often taken over by system functionality and never given to the app in the first place.