What makes the birds happy
Bird feeders have seed in them, bird houses have birds in them! Hellooooooo!
As long as the entrance/hole is a proper size and the birdhouse has ventilation openings without letting in rain, the birds will be happy.
Unfortunately many bird houses not approved by the National Audubon Society (usually the small, painted “cute” looking ones) have no ventilation. The birds nest, lay eggs, the eggs hatch, and then the babies die because they overheat.
You don’t want to make the birds “comfortable” by adding stuff for the birds to nest in, because the house will then smell like a human and the birds won’t want to nest. They’re quite content to find everything they need to build a comfy nest inside all on their own.
Happy Birding!