Just like on the desktop there are many parts needed to make a fully functioning operating system. That begins at the bootloader and kernel and goes over some kind of window manager or desktop environment up to the actual programs running on the hardware.
And all of these parts may need more or less adjustments to work on the smartphone form factor. And like on the desktop there are different projects trying to work towards that goal. And of course they work together where applicable. But like on desktop some projects might have different goals so there might be two similar projects to provide the same part of the software stack.