Electricians. Top skilled folks for the most part who can do industrial level conduit work and the type who can operate switching gear and control systems. There is enough of this ongoing work for these huge facilities to effectively employ a half dozen full time contractors or more. One of the facilities I work the most in has electrical contractors on-site every single day, with at least a few trucks in the parking lot. These are local union guys. Always something breaking, needing maintenance, or a new area of the facility being refreshed. The facility is over a decade old and the work has never slowed down.
Plumbers. Cooling these facilities takes vast amounts of plumbing work. And it's also typically some of the highest skilled plumbing needed outside of refinery and other manufacturing plant work. When you have 50 giant chillers running 24x7 at least one is undergoing some form of maintenance at any given time.
Probably overlapping with the above, but HVAC technicians. Again, the scale of these facilities means constant work being available as you are operating at miniature city sized installations.
Security guards of course. Not really material though. I've noticed more armed guards than before, with at least two on duty 24x7. As these places get more controversial, I imagine this sort of staffing will increase.
On-site (IT) technicians. For facilities these sizes, you will be staffing it 24x7 and have a large enough crew to get basic refresh projects done. Hard to really estimate this, but in the dozens of full time labor for these giant projects. Think folks who can pull cable, troubleshoot basic hardware, swap drives/bad RAM sticks, etc. For the larger refresh projects contractors typically get flown in during a surge so on-site staffing is relatively minimal, but very few facilities are operating "lights out".
Then you have facility management - highly skilled positions that know how to operate all the electric/mechanical and cooling equipment during emergencies. Every facility I've worked in is staffed by a crew of around half a dozen of these folks or so, with the top tier subject matter experts being flown in during critical emergencies. These are the guys generally coordinating all the contract labor above.
Probably a couple mid-tier network engineers and higher skilled sysadmin types as well depending on who is operating it. Everyone loves to pretend these are highly automated and copy/paste facilities hyperscalers are just perfect at executing - but there is a lot of "dirty" hands-on work to be done since that stuff is not nearly as perfect as advertised and often requires hands-on problem solving and on the spot hacks to get stuff going. As anywhere, how the sausage gets made is a lot uglier than the marketing.
Once you get out of the highly competent hyperscalers, the above numbers go way up. Enterprise datacenter operators are going to need far more on-site labor due to simply not being great at this sort of work. The stories I hear of some current builds are rather humorous in how many people it's taking to get stuff working - basically solving what should be automated via manual processes.
It's not a lot of jobs, but for these huge 100's of Megawatt facilities the low-end is probably in the 100+ range of FTE equivalent labor after construction is completed. Everyone but security and the basic "remote hands" type employees would be in the $100k+ salary range.
