It bears repeating.

One of the cheapest ways to build depth in a TTRPG setting is to reuse things. Need to introduce a NPC? It was the one that the PCs ran into a few sessions ago. Need a temple? Use the one that they went to get healing from before. Want to reinforce a theme of some kind, say oppressiveness? Have them get stopped by the guards. Again.

That is it, really. The rest of this post is some examples and expanded thoughts, because I can not leave well enough alone, and so I need to over-beat the egg with more words.