Its pretty simple I suppose. They shut down the liquid side valve and turn on the compressor. Now compressor would start sucking in the refrigerant in lineset. It doesn't take much time. Have some pressure gauge attached to charging port to measure pressure (installers don't even bother to do that, they simply make use of their judgement and compressor sound). After a while they shut off the gas side valve as well and shut down the unit. Its better to have a pressure gauge attached so when pressure drops to zero, you are somewhat sure that all refrigerant has been sucked in by compressor. A small consideration here is to ensure that while connecting the pressure gauge, the hose must be purged too else compressor will suck in the air too that's in the hose (It was in my mind but simply forgot to tell them).
No, they didn't charge any additional refrigerant from the cylinder. The low side pressure already touches 150 psi so no indication to suggest there is any shortage of refrigerant.
