Create a default Tilemap, from the GameObject menu (menu: GameObject > 2D Object > Tilemap). For the Hexagonal and Isometric Tilemap options, please refer to the documentation on their respective pages.
After selecting the Tilemap option, a new Grid GameObject is created with a child Tilemap GameObject in the scene. The Grid GameObject determines the layout of its child Tilemaps.
The child Tilemap is comprised of a Tilemap component and Tilemap Renderer component. The Tilemap GameObject is where the Tiles are painted on.
Create additional Tilemaps by selecting the Grid GameObject or the Tilemap GameObject, and then go to the menu: GameObject > 2D Object > Tilemaps, or right-click on the selected GameObject and go to 2D Object > Tilemap
A Tilemap GameObject is then created and added to the Hierarchy as a child of the parent Grid GameObject, even if the sibling Tilemap GameObject was orignially selected.
Once the Tilemap(s) are created, adjust the Grid property settings to adjust the properties of its Tilemap children. Changes in the parent Grid GameObject affect all child GameObjects with the Tilemap, Tilemap Renderer and Tilemap Collider 2D components.