Tilemaps are a quick and easy way to make well-made 2D level designs while saving a huge amount of time. The process of creating tilemaps begins with creating a tile palette in an art program, importing it into Unity, slicing the palette into sprites, and then setting up the palette in Unity. I have created a tutorial on tilemaps with some explanation on this process.
There is no easy built-in way to make 2D colliders interact with 3D objects. Instead, we have our own custom component for this, regarding tilemaps. This component can simply be applied to a given tilemap, and will automatically generate a mesh collider out of it upon initialization.