> how do I maximise a window on macos with my keyboard?

System settings > Keyboard > Keyboard Shortcuts > Window > Fill. Default on my machine is fn+control+f, but you can also reassign that there of course (which AFAIK is something Windows doesn't let you do, by the way.)

> I end up using Rectangle on macos for moving windows and maximising them using keyboard shortcuts

I also used to use Rectangle, but by now the built-in window management shortcuts fulfil the same purpose out of the box (almost, that is; where Rectangle can move Windows onto the next screen, that is arguably where the built-in shortcuts fall flat, only being able to arrange on a single screen)