What assignment do military men on deployment do?

Depends on their MOS. They may cook for the rest of the troops, treat the wounded, search for enemies, fight the enemies they find, fly the helicopter and transport the troops, use the gun mounted on the helicopter, repair the helicopters or other vehicles, make sure the troops have enough ammo, use the artillery gun, use the tanks, operate the nuclear reactor if they're on a ship or sub, and lots more. Or, as Mrsjvb said, their assignment might not be relevant to their MOS.

My last deployment, I was the Motor pool chief for a unit at a firebase in the Afghan mountains.

Pretty much the same thing I did when stateside.

Clean  and clean 

There is a HUGE variety in this.  For my career, while deployed to Alert the work was much different from going to sea.  Or to Germany with the Army.  Of Bosnia, or Afghanistan.  Each place was different, with it's own assignments.

I think there is a test that determines what kind of job you will be given based on talent/ability.

I grew up on base, but I never served.

I did sit in an F-15 once, though. 

The same job they did when they were not deployed. 

A lot of the same jobs that military women do (I don't know if you are aware that the military hasn't been all male for a long time).

Seriously, there are thousands different jobs in the military.  Obviously an Army specialist MP in Afghanistan is going to be doing a different job than an Air Force major B-1 pilot in Qatar.  

Whatever they are assigned, which may or may not have anything to do with their MOS