The only ones I have seen are where the casing iscovered in drywall the problem is you need something for the hinges and lockset to be attached tothe only way to totally eliminate this would be to go with barn door type install , it would work on interior doors but a bit of a problem for exterior.

You use metal casing made especially to hold the door and be mudded in. But unless you have a contractor in site with experience with this, planning something like that isn't going to work. It takes a ton more skill and labor, and consequently is very high in costs. And that's for someone experienced. Inexperience costs more as you have to redo it with your fallback plan.

The company I am using is called EzyJamb.

There are two ways to do a swinging door without trim. One is to install the jamb have it completely plumb, level and square, and of a depth that is the thickness of the wall minus the thickness of the drywall. The drywall then has to be beaded and mudded to the jamb, often with a kerf (slot) in the jamb for the bead to be inserted in the jamb. There is a lot of labor involved, and the person really has to know what they are doing.

The second, as live-wire-oak mentioned is to buy jambs purpose built for trimless doors. EzyJamb is the only company available for residential that I am aware of, there may be others. It is a heavy metal jamb assembly. I do not like the standard hinges which are surface mount on the metal jamb, I went with a concealed, Soss-type hinge (RocYork brand) which is an upgrade per hinge.

It still takes precision to install it, but it is a big labor saver in terms of installation time. However, they are expensive compared to a regular jamb or prehung door.

My assemblies are pricing at $500 per opening for the jamb assembly and about $150 of that is hinge upgrade. Here is one installed. This is the hinge side, so they are completely concealed.