Okay, it's a staple amongst certain fantasy settings where you have different races of creatures and beings co-habiting, which leads to certain off-shoots of different races (the half-elf and half-orc perhaps being the most commonly known).

But how feasible is such a thing? Presumably these different races are all significantly genetically different from each other that a viable mixed race would be biologically impossible? Sure, different races with sexual organs that fit each other would be able to enjoy physical relations (if they were into that kind of thing) but would they be able to biologically produce offspring?

Especially when different races seem to have natural, inherent biological traits: elves are typically very long lived and can see in the dark etc.

And then there is the notion of "half-human/half-demon". Not only are the races separated by their genes but also their planes of existence!

In terms of science, would this be possible?
(yes, I know that asking for a scientific explanation for a trope in a fantasy setting may be somewhat backwards but I am curious to know).

Specifically, this is for a fantasy type of world that I am building. There are three main races: human, alfar (something akin to elves) and floring (a kind of Halfling), with the possible inclusion of a race of infernals that are a line descended from a mixing of human and "demon".

I hope that makes sense.