Drop in your world folder and the app will handle the rest: find level.dat and every player
in playerdata/, then build a repaired level.dat for each one with their
position and inventory — no manual NBT editor required.
The world folder that has level.dat directly inside it — e.g. the unzipped folder you just downloaded from a hosting provider.
On Bukkit-based servers (like Spigot or Paper), the world is stored in 3 separate folders for each dimension. Use only the primary world/, not world_nether/ or world_the_end/. Because of that, to actually play it in singleplayer you'll also need to convert it to vanilla using our BukkitToVanilla.
A server keeps every player's data separately in playerdata/, keyed by UUID. In singleplayer only one player is tracked, baked into level.dat, so download the level.dat for whoever is actually going to open the world in singleplayer. Everyone else gets their own inventory back automatically when they join over LAN or a LAN-tunneling tool, matched by their UUID in playerdata/ — you don't need their file for that.
level.dat with the one in the folder you uploaded here — make sure it's named exactly level.dat.