Minecraft wiki entry for NBT_format, associated text: level.dat is stored in compressed NBT format.
<player>.dat files are stored in compressed NBT format.
idcounts.dat is stored in compressed NBT format.
villages.dat is stored in compressed NBT format.
raids.dat is stored in compressed NBT format.
map_<#>.dat files are stored in compressed NBT format.
servers.dat, which is used to store the list of saved multiplayer servers as uncompressed NBT.
hotbar.nbt, which is used to save hotbars as uncompressed NBT format.
Chunks are stored in compressed NBT format within Region files.
scoreboard.dat is stored in compressed NBT format.
Generated structures are stored in compressed NBT format.
Saved structures are stored in compressed NBT format.