I worked in Surveillance in Las Vegas for a couple years – didn’t like being a snitch and moved on.
Some very small casinos may only have part-time observers, perhaps only a couple of shifts per week, with somebody (perhaps a security guard) assigned to change the tapes every eight hours. (My experience was over 20 years ago, everything was tape … I don’t know if they have since moved to digital recordings.)
Big casinos have full-time observers – perhaps two or three per shift, sometimes only one during slow times. Knowledge and experience of Surveillance Observers vary widely: some were former security guards, some took the class at the community college, some may have worked at other low-paying jobs in the casino like the gift shop, some were retired cops, some were friends of the boss, a few were former card counters who had been barred from play, and only a few (like me) had casino floor experience.
You mainly watch one working monitor directly in front of you and occasionally look around at a bank of a few dozen other monitors. You can bring up any camera on your working monitor, so you can watch or zoom in (to see the hairs on their knuckles) any game or area of the casino.
Observers don’t only watch blackjack games; most of them spend much more time watching slot areas, bartenders, other casino employees, or just generally scanning the floor looking for signs of potential trouble. One guy I worked with tended to just look for a guy with a baseball cap pulled low over his eyes doing a rubber-necking look-around thing, he’d follow that guy on camera and often caught purse snatchings, money grabs, slot sluggers, and other low-life petty crimes.
All the cameras are capable of recording but not all of them are constantly recording. Some key areas will be constantly recorded but things like blackjack games may only have a camera (or three) on them when there is big action or when something looks suspicious to surveillance personnel or somebody from the pit has called and asked for the game to be observed and recorded.
Surveillance is not a particularly high paying job, but is above the minimum wage paid to Security Guards. Surveillance people eat alone; pretty much nobody wants to be seen associating with them. It’s a good fit for people who don’t want to work as part of a big group and who get a kick out of catching bad guys. If you learn to be good at it, you end up spending a lot of time in court as a witness.