IME/IMO, once a maker gets to a certain output volume, they tend to bring the heat treat and tempering process in house. It all depends on when they can justify the equipment expense. It shortens their time from start to finish for a blade as well. Since they no longer need to send blades out to another location for the process. Plus they gain additional quality control over the final product.
As already mentioned, some makers will be over the top in how they perform the task. Others will be less stringent on things. Generally speaking though, the temperatures involved are pretty set (small variances are allowed) for the hardening process. Quench methods depend on what steel is being processed. You won't process O1 or 5160 the same as ATS34, D2 or one of the newer stainless super steels. Some steels also benefit from a cryogenic process where others either won't, or it won't be enough of a benefit to justify the additional steps.
From what I've seen of things, over the decades, the heat treat ovens have gotten a lot better, and might not be as crazy expensive (haven't looked in some time). What happens to the steel is a bit interesting during the different stages. Tempering a blade is a critical step too. Since if you don't, then the result is something very hard, but brittle (easy to break/damage). Often the desired hardness numbers are determined by what the blade will be used for.
All of the needed information is typically published for an alloy. What temperature to harden at for how long, followed by what quench method is needed. Then they'll give what temperature to temper the item at (also for how long IIRC) to get to the desired end hardness.
BTW, edge retention can be ruined if a blade edge is ground so that heat is created/transferred to the edge. This is one of the reasons I'll never use one of the belt driven sharpening items (worksharp). You have to be very careful when using such items in order to keep the edge cool enough to NOT screw up the heat treatment that's been applied. I'll gladly spend the time to sharpen my blades with the setup I have knowing that zero heat is going to the edge during the process.