Anthony
'Typically' in a sim detail where I work/am tested; the ATC is provided by the trainer running the detail. The trainer will also act as ground crew, cabin crew, company ops etc.
If you are doing recurrent TCAS training, then the profile flown tends to be pre-programmed into the sim, so that the sim generated warnings are 'genuine' and wothout human input. This gives the trainer the oportunity to distract you prior to the event to get your 'true' reaction to the TCAS event.