Just a quick test of workflow with no particular goal in mind other than to create a short sequence with moving background (sky), middle ground (set), and foreground character.

I suppose if there was a goal it was to set up the scene so the character had to move over the top of the set at the first but then move into/disappear behind the foreground elements.

In traditional animation this process might require registration of character with background in order for everything to work but digital workflows make the process simple and straightforward simply by ordering layers.  While not used in this sequence OpenToonz can animate the Sort Order so that things that are on front move back in depth.