As a user land developer, you can have glibc or musl, initd or systemd. Is dbus being used? They all work differently.
I would expect Windows RT and Windows CE to somewhat differ too. Despite being NT.
Why would an application developer care about the init system? Start scripts and units for demonized stuff can be honestly made by users and maintainers, if that's expected to be packaged. If it's not, it's half an hour of googling to make functional enough ones for most purposes.
DBus is such a common thing that there are applications not working without it running, and nobody really complains. You can assume it is, or you can ignore its existence. That's changed by installing\uninstalling DBus. Not a difference between two operating systems, LOL.
glibc or musl - yeah. Different enough. Still the OS is the same, can use a musl chroot from a glibc system. Can use as many chroots as you want.
No way. Someone is looking into an open window. Wow.