This uses TileMapLayer node in the scene to spawn your items (they must be prefabs as packed scene .tscn files). You just need to decide which cells spawn items and add them to the array provided by the plugin. The plugin can spawn randomly in those decided cells or can be used to spawn in all of those decided cells location. The plugin provides freedom to create a class or different groups to spawn like weapon items, casual items, etc. You just need to be careful about the spawn location of two different classes as items can't be spawned at the same cell. The plugin provides an option to list the items per group and also decide drop rates or spawn rates; if set to 0 then it spawns equally between items in that group.

For more details, read the readme file that is provided with plugin. You can also read this git readme link: https://github.com/WarDragon3399/spawn-manager