Your cap looks like it traps some water like the S-bend or P-trap or whatever it's called in your area, to prevent gasses and smells coming up out of the pipes.
Yes once the cap is installed into the white " female " part, it makes indeed for an 'S' shaped chicane.
to prevent smells ?! Hmmm..... didn't think of that ! I have an extremely poor sense of smell in general, so most odours that annoy people, I can't even detect at all.
OK maybe I better put that cap back on now, and just pop it off to give the white part a good clean every time I notice the flow is getting slow. Now that I have a diagnosis and a quick and easy and cheap fix / solution, I am happy. No need for an endoscope and no need to spend fortunes all the time for cleaning products...
Sanding / wood : while at the H/W store I had a quick look at the small sanding machines.... boy even the small ones cost a lot ! 150 Euros for a decent brand !
The cheap ones are like 35 to 70 Euros, still quite a bit for the crap it is... it's not like a sanding machine in 2022 was some rocket science....just a crap chinese motor in a crap chine plastic case.... come on...
Too expensive so will wait for my first pay check to show up on my account, if I make it that far that is.