If I get the Xbox Ultimate Gamepass, can I use the Minecraft launcher on Macintosh to buy...

If the launcher is available for the Mac, then you won't need to purchase Java edition, as the game is included with Game Pass Ultimate anyway. If you log in with the account you got ultimate with, then you can launch the Java edition from the launcher.

I open the minecraft launcher, log into my account, and then doesn’t let me play minecraft, i can use the minecraft launcher in the xbox app using gamepass, but cannot play minecraft

Which gamepass are you talking about

Gamepass for PC OR gamepass standard

If you are using gamepass for pc for access to Minecraft on pc and using the same account on console To share then no

As gamepass for pc wont work on console

You need gamepass standard or ultimate for console

If you want to share a subscription for both pc and console you need ultimate which includes bothpc and console titles

Another solution since pc doesn't need a subscription other then realms

Is buy Minecraft on pc and keep realms on your profile and buy gamepass standard for minecraft access on console or buy core and Minecraft

Its on pc and its on the actual Xbox launcher, I have gamepass but it keeps saying to join gamepass (Ultimate)

I have already migrated to Microsoft, and i do not own minecraft on the Xbox, as it is available for free with GamePass Ultimate. Do i HAVE to buy it now or something since the merge??