......how do they clean it out at the end of the day?

Solvents won't clean the water out of the nooks and crannies?

What process, or products do they use?

First you rinse it with clean water, then take it apart and rinse it again.  If you can you blow it off with compressed air (depends on what's available to you at time)you do, then you dry of the component as best you can with a dry rag or towel or just air dry depending on the climate, then you clean it as you always would.

Well  when you get out of the water tip the barrel forward and let the water drain out.  Then tip the rifle back so the muzzle is up to the sky and pull the charging handle several times letting the water drain out of the little hole in the buffer tube.   Then clean as normal as soon as the tactical situation permits.  Hope this helps.  By the way if you do submerge your rifle, compressed air helps get all of the water out faster.

Remman