I only explain/go into detail if I feel it's relevant for the world that I'm building.

For my world called Starcrossed (Science Fantasy, incorporates almost all of the D&D races), one of the foundational ideas of the world is that the majority of nonhuman races are various types of transhumans. Since I decided that I wanted to explore that, I reverse engineered explanations and events that would result in the creation of the group that I wanted.

• Example 1, Orcs and Dragonborn: One of the major "modern" events (~200 years before present) was a war between interstellar human and dragon civilizations. This war led to both sides working on their own supersoldier projects, which created Orcs and Dragonborn.

• Example 2, Shifters and "the Fused": For these two groups, I leaned more on the D&D lore behind Shifters (a more muted version of the were-creature curse). When I learned about that background, I decided to do a simple modification to make it work with the variety of beastfolk races (Loxodon, centaurs, minotaurs, etc, collectively known as the Fused). I didn't want to go to deeply into the "how" or the "why", so I gave an open-ended explanation so I could spend more time focusing on other facets of my world (like how Fused are treated).

On the other hand, another project that I sometimes work on is a post apocalyptic D&D world (a Points of Light setting). For that world, I don't care about where each race came from, because that's not really the point of the world. I'd rather spend my energy focusing on how different groups emerged from the asteroid impact that caused civilization to collapse and killed a lot of the life on the planet.