Hi,
first of all L 900 is an autofocus enlarger, hence some differences between him and for example L1200.
Those two nested knobs - inner one is just locking/unlocking the heads travel (you just push/pull it) and the outer one moves it when the lock is on.

Focusing is entirely another matter. You have the big button at the bottom of the collumn called focusing variator (lock is on the left, actual variator on the right) and both lenses are set in metal rings (zebra stripes on mine) that can be turn around. Idea is that you calibrates the focus for both lenses with the head down, check or recalibrate it with the head up (possible more iterations) by those two metal rings. And the focus variator is adjustment in case you use some printing frame and project your image higher than on baseboard. If you set it all up, then voila, inner mechanism of this baby makes your pictures focused all the time.

I have been struggling with all of this during spring so I hope I could help you a bit. If it is of any use I have some kind of manual in german, could upload it to somewhere.

Cheers,

Jan