You mean why we can say both Har du något paraply? and Har du ett paraply?
Maybe the question ought to be about why you can't say Have you any umbrella? in English. In the plural, it works the same in English as in Swedish, you can say both Do you have umbrellas? and Do you have any umbrellas? (Har ni paraplyer? and Har ni några paraplyer?)
The singular with 'any' tends to work well with abstract things, but not so well with physical objects (but it's probably even more complicated than that). – There's a difference in meaning between the two different versions in Swedish too. Har du något paraply? is somehow more hypothetical and 'open'.
Like many here, I'm also having a bit of a hard time wrapping my head around determiners. In this sentence, is the idea of 'något' more of 'any umbrella' or 'some particular umbrella?' Or am I just off-base with the question? Like, even though umbrella is a concrete noun, is the concept of 'any' at work here, kind of like how in English one can ask if a person has 'any idea?' To me, that is asking ' do you have (at least!) one of a variety of potentially existing ideas?' So, here it'd be 'do you have one of a variety of potentially existing umbrellas.' Or, is it asking if the person has a particular umbrella, like maybe their very own umbrella? Thanks!