Birds have all sorts of ideas about home sweet home. Take a look https://www.dkfindout.com/ to learn more.
You can also read the RSPB web site How Birds Build Nests? page to learn more.
Our houses are all different, and it is no different for birds. Some birds are larger and so need bigger nests, others need to keep their nests hidden from predators( like other birds or animals which might eat the eggs or babies).
They build nests to be home to their eggs and baby birds (nestlings) when they hatch. So it is all about camouflage (blending in), shelter, and safety.
Birds also think about the weather when they are nest building-they want to keep them as dry as possible, they don't want the wind to blow the nest down, or the nestlings to get too hot or cold.
The grass or leaves or moss or animal fur is like a cushion for the eggs to keep them from breaking.
This is part of the Birds information.