It’s better to build the jamb around the door before installing. That way you know it’s true and fits the door. It’s relatively simple, you can go about it with basic screws or fancy joinery depending on your skill level. I built a custom jamb last year to reuse an old door original to my home and I was surprised how well it turned out and people are amazed when I tell them I built the door jamb. For reference, I’m a slightly skilled DIYer who is happy to take on a project slightly above my skill set to learn.