I think you have concerns about how this time off will be taken at work and only you know if they have reason. BUT they need to ask you questions to understand the nature of your condition, the surgery and therefore the implications for them so don't be worried by the questions. That’s it. It’s not an interrogation, it’s just HR ticking boxes. You have been there 8yrs, they will know what you are like as an employee.
A catheter at home is not a minor thing, it affects mobility, sleep, infection risk, and comfort. Add abdominal or renal surgery on top and six weeks is actually on the conservative side. What they can’t do is override a fit note or pressure you to return early. A GP’s note is the legal evidence of unfitness for work and honestly, managing a catheter at home while recovering from kidney/bladder surgery is not compatible with most jobs, even desk‑based ones. Painkillers alone can make someone unsafe to work.
So the reasonable position is: You’ve had major surgery, you’re recovering with a catheter, and your GP has signed you off. Your employer is simply gathering information, not challenging your absence.
If anything, you need to rest properly so you don’t end up back in hospital. And that includes stressing about what work thinks. If you think it will help you to get the fullness of your situation across to work, email HR or your line manager telling them the surgery you have, that you still have a catheter and stent in and that you're on strong painkillers hence the fit note for 6 weeks. Tell them that as soon as you have your next check up, you will update them but expect to be off for the full 6 weeks. And stick to it.
I have recently had some similar surgery myself and I wouldn't be surprised if you are off longer but you just need to concentrate on you and as you are no good to anyone is you are dead! Big hugs.