Actually I was using macOS on my main desktop until recently. If you care to read it, I described the reason behind here: Praise for multitouch trackpad (Touche) in Zorin 16
My system is not made by Apple, but I assembled it to be 100% compatible with macOS.
Now to your questions:
You can use Startup Applications for that purpose.
If you uninstalled macOS, there is no way to access any application in macOS.