Concepts has two layering modes - Automatic (which sorts by tool) and Manual (which gives you manual control of which layer you draw in). The default setting for layer management is Automatic mode, where the active layer is determined by the tool you’re currently using. This makes it simpler to organize and edit your work after drawing without having to think about assigning layers beforehand.  Your tool layers are sorted in the following order, from top to bottom:

You can change the order manually by drag+dropping layers above or beneath each other. Learn more about this in How do I change my layer order?

The Masks always mask in the current layer. The Slice and Nudge tools can edit linework either in the "Active" layer or in "All" layers at once. Just tap the Layers toggle at the bottom of the screen when these editing tools are active.

If you want to draw in a different layer, either change your tool to stay in Automatic Mode, or open the Layers menu on the canvas and just tap the layer you’d like to draw in. This will activate Manual Mode, which allows you to draw with any tool in any layer, and create multiple layers of the same type of tool.

For an illustrated tutorial on layers and how to use them to organize your sketching, check out How to Use Layers to Take Notes, Illustrate and Design.