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Thank you, Marc, for your reply.

Actually, Ctrl+Tab is exactly THE shortcut I like to use in any software for tab navigation (Firefox, Safari, Terminal, etc.).
I'm using musescore on a mac in a french environment (french macos).
Ctrl+Tab in musescore does nothing.
In the Preferences > shortcuts, looking for "partition" (which means "score" in french), I found the default shortcuts (see picture). Thank you!
In french, "Partition suivante" (resp. "précédente") means "next score" (resp. "previous").
And, on a mac, the "⌘" key is called the "Command Key" (abbrev. "cmd") cf. https://en.wikipedia.org/wiki/Command_key

Imho, these default shortcuts are either silly either full of bugs (and probably both)!

Let me start with the "cmd-tab" (resp. "cmd-shift-tab") shortcut, which is supposed to be bound to "next score".
Under macos, it is a global shortcut, which is bound to "next application" (resp. "previous"). It is the equivalent of "alt-tab" on a PC/Windows to switch from the current application to the next one (resp. previous).
This shortcut is not supposed to be changed to be local to any application. This is why many applications use "ctrl-tab", rather than "cmd-tab".
If you type "cmd-tab" in musescore, it just switches to another application (Firefox, for example). If you try to bind it to some function in the musescore Preferences > shortcuts, it just switches to another application and cannot be bound.
So "cmd-tab" and "cmd-shift-tab" really silly.

Let me continue with the "cmd-}" shortcut.
If I switch to a virtual "American keyboard" (I mean, the software which binds my physical keyboard to the generated characters (in the mac Preferences)), then in order to get the "}" character, I have to type "shift-]" (on my mac french physical keyboard, it's the "$" key, just before the "return" key).
If I type "cmd-}" (which corresponds logically to "cmd-shift-]" (or physically to "cmd-shift-$" on my french keyboard)), then musescore switches to the next score. Perfect!
But if I switch back to a virtual french keyboard (which binds what I see to what I get), then in order to get the "}" character, I have to type "alt-)" (it's the key just after the zero "0"). But, in musescore, "cmd-}" (which is obtained with "cmd-alt-)") does nothing. Bug #1.
BTW, I just tried "cmd-shit-$" (with a french keyboard) and it switches to the next score! Ok, I got it, but it's bug #2 since the described binding is "cmd-}" and not "cmd-shift-$" (which should be notated as "cmd-", since "shift-$" produces a "", as you can see on a mac french keyboard).

Let me continue with the "cmd-{" shortcut. (and let me be a little bit more concise...)
With an American keyboard, it switches to the next score, but should switch to the previous score. Bug #3.
With a french keyboard, "cmd-{" (obtained with "cmd-alt-(") does nothing. Bug #4.
BTW, with a french keyboard, the American "cmd-{" (obtained with "cmd-shift-^") does nothing either.

Now, let me change some musescore shortcuts.
If I try to enter "ctrl-tab" or "ctrl-shift-tab", then nothing happens. It seems musescore does not get any input shortcut from my keyboard. Any combination of "ctrl" "alt" or "shift" with "tab" does nothing. Bug #5

If I change the "previous score" to some other binding (not in conflict with any other command), like "cmd-D", then "cmd-D" switches to the next score, instead of the previous score. Bug #6 is probably the same as Bug #3.

But I like musescore anyway... ;-)