Thank you! If you know the diameter of the rim (the round metal part in your description), e.g. 600 mm, and you can assume a tire width, e.g. 40 mm, then you can search for "40-600" and find some tubes:
My problem is that I have a rim that is there for good, and then I would like to see which tires are possible for that given rim (there are several choices of width and I do not understand what drives the minimal and maximal width - I can imagine the the brakes are a limiting factor for the maximum width, but maybe this maximum is smaller because [some measurement of something]).
Then comes the tube, thus the OP's useful page.