Instrumentation is field-specific. It really depends how bespoke your requirements are. Some places buy stuff off the shelf, others fabricate in-house.
In astronomy it's called instrumentation. Astro has relatively sane nomenclature. You have observational (looking at stuff and gathering data), theoretical (theory and simulations) and instrumentation (building stuff). Everyone in the field understands those terms. That said, instrumentation spans everything from construction, design, calibration/characterisation, etc.
In other branches, as the OP pointed out it may be "experimental" physics. Problem with that term is it's extremely vague. That's essentially anything which doesn't involve theory or simulation - lab work. That doesn't necessarily mean you actually build anything though. I was surprised at how little we got taught about instrumentation during my physics degree. Of course we had labs, but it was more about using kit than how it was built. It's almost as if it's someone else's problem, even though that's not how research actually works.
The irony of physicists sneering at engineers is that most labs employ a bunch of them who do most of the actual design and build work. They tend to be less focused on publication, but they are absolutely critical employees. The sorts of people who've spent 20-30 years working on some uber niche detector tech and know more than the people writing the papers for sure. A good chunk have physics PhDs.
When you get to big money stuff like particle accelerators and telescopes, a lot of this is contracted out. Physicists and engineers are responsible for designing the spec, but it's less tinkering in a lab with hardware. In industry it's mostly called engineering (e.g. optical, mechanical, electrical), but I've seen engineering physics too. That tends to be in companies that build very niche equipment specifically for physics research.