We don't really go into depth on the lore of how interbreeding works - deliberately so. However, even though most of our 'half-' races are treated as half human, that's really just the default, because humans are randy buggers.
Halflings are their own species, not a half breed from anything else - though there is the cultural rumour that suggests that stronghearts may have a touch of dwarf somewhere back in their historical ancestry.
The general ideology follows tolkien-esque shapes; dwarves, for example, are usually considered to be largely incompatible with *most* other species... but these aren't at all defined rules. It's common also to presume that halflings are generally not considered to be compatible for offspring with most other races as well.
In all cases, if direct biological compatibility is extremely low viability, or not viable at all, then there's always magic and divine intervention. Praying to your fertility/family deity of choice can and does result in divine 'assistance' with making things work that otherwise wouldn't, if you're lucky. There are also magic items that crate compatibility and conception where it otherwise would not work (such as the beast-mother's mask from older editions).
Dragons (of either type), full celestials and full fiends are generally considered to be able to procreate with any mortal race, if they wish to.
In terms of couplings producing offspring that isn't one of the more commonly viable and compatible crossbreeds, the usual outcome is that the child will be more or less entirely of the mother's race, but will bear hallmarks and distinguishing features and traits of the father's race in some fashion.
As always, all of this is just common practice and generally used ideology - none of it is firmly defined or set in any kind of canonical stone.