The bugs themselves are really easy to kill! Spray them with rubbing alcohol and they're dead. They walk over diatomaceous earth and they're toast. Reach 115 degrees for 30 seconds and they're done. But yes,
infestations can be resilient because they scram at the first sign of trouble and hide really well if they're forced to live somewhere other than your mattress/box spring/bed frame. They will hide in electrical outlets, clothes, cracks in trim, in furniture, under carpets, move to different apartments, etc. However, they need a reason to leave your bed-- e.g. the mattress is too crowded with other bugs, has been treated with insecticide, etc.
The situation you had in your apartment would never have gotten better. Without treating the whole building, spraying insecticide will just push them into adjacent apartments. Many jurisdictions distinguish between infestations local to one dwelling to be handled by the tenant and infestations that are building-wide which landlords have to treat. If they just keep pushing them around, they never have to shell out for a building-wide treatment and they always look like a hero because technically they didn't even have to pay to get your apartment treated.
Step 1 is to clean every tiny little crevice of your bed frame, bedding, box spring, and mattress and if possible, get a mattress bag.
Step 2 is to put bed bug traps under each leg of your bed-- they can't jump or fly, only climb.
Step 3 is to make sure no part of your bed or bedding is touching the wall or floor.
Step 4 sprinkle diatomaceous earth around the base of your bed and the edges of your room.
That's actually enough to stop you from getting bit right there and if your infestation is really minor, is probably enough to entirely eradicate it. Unfortunately, they can go 70 days without feeding, so you'd need to keep that diligence up for quite some time. That's why a proper extermination will be building-wide and happen at least twice with the second treatment being after the eggs should have hatched. The best method, IMO, is tenting the building and heating it to 115 degrees for a few days. Not cheap, though, and obviously very disruptive.