About Tilemap in the EditorEdit
The Tilemaps make up the basic flooring of your map, it's function is to be just a static layer that can be rendered efficiently and can be used in combination with other Tilemaps to create more detailed layouts for the map.
Simply select a Tilemap on the side menu and configure the brush size you wish to use.
Left click to paint down the tiles and right click to remove, you can only remove tiles of the same type you have currently selected so don't worry about being precise since Tilemaps can overlap each other.
Some Tilemaps automatically create bordering tiles such as 'a_dirt_scattered' and are designed to give a better level of detail to put over the top of other tiles.