It can happen both ways. My PhD ended up being a computer vision project and while I could program better than most new grad students I didn't have any signific

It can happen both ways. My PhD ended up being a computer vision project and while I could program better than most new grad students I didn't have any significant experience with image processing, visual geometry or CV (or ML for that matter). I did know that going in, but at the time I was more interested in working on instrumentation (which I got to do as well). As a PostDoc I flipped the other way, after a couple of years in industry I joined a group where I brought some specific expertise to the group that they didn't have. Good strategy for getting work in my experience. This is fairly normal: you get some funding to do a project and you have to hire someone with experience, though in this case the PIs were experts in the domain of application. That's reasonably common for cross-discipline projects, but in this case you really have to trust that your hires know what they're doing and you need to be able to assess their work without deep knowledge of the techniques used. PhDs can be absurdly specific and many are complex enough that you can't really study them until grad school. Many universities don't teach quantum computing, but as a physicist it's probably assumed you have enough of a background to understand the material.