General :: Writing A Kernel Module And Make Concepts

i have written some really basic loadable kernel modules. There are a couple of concepts that i'll like to get cleared.

In a tutorial, the Makefile was given as follows-
obj-m += try.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

I understand that -C option of 'make' changes the current directory to /lib/modules/$(shell uname -r)/build (which depends on my kernel).

Now, my question is why to change to that particular directory? Also M=$(PWD) will hold the dir where i have written the module. Why is it required? What does 'modules' at the end of that line indicate?

Q2> We need to include <linux/module.h> and <linux/kernel.h> in a module. If i am right, the module uses kernel headers for these definitions. But how does 'make' find these definitions.

Q3> We are planning to write a custom scheduler for linux kernel. cat /sys/block/sda/queue/scheduler gives us the currently loaded schedulers. How do we expose our scheduler to linux kernel so that we can use it by switching to it.