Sometimes the brush button paints the selected asset, other times it does nothing.
I discovered the padlock was locked this one time, what is that for?
What are these transform numbers in the tile asset?
Where do you actually paint? brush in the scene view does nothing.
What interface do i use to define how the tiles interconnect?
This quick Tilemap Basics Video can help you get started! The updated Tilemap Manual has more details and links to items like how tiles can interconnect and determine the correct Sprites to use.