[RPG] Do subrace benefits add to or replace base race benefits

You have to choose one.

Subrace is a trait from the dwarf race, with the following description (PHB 20):

Subrace. Two main subraces of dwarves populate the worlds of D&D: hill dwarves and mountain dwarves. Choose one of these subraces.

And yes, you get both bonuses (so +2 Constitution and +1 Wisdom).

(Note that there is a third official subrace available in the Sword Coast Adventurer's Guide, the gray dwarves, also known as the duergar (SCAG 104). This subrace can cast some spells innately, but suffers from sunlight, like the other Underdark races.)

No, it's added to your score first.

Your racial modifiers add (or subtract, in a few weird cases) to the ability score, which you use to find your ability modifier.

So, like, if you have that Con 15 and your race gives you +2 to Con, you have a Con 17, which you use to get the +/- that gets added to every Con roll.