Thanks for your feedback !
It's easier than you think. Unity is a 3D engine, even when you make a 2D game, it's in fact 3D. So all the sprite have a 3D position. The 2D effect is due to a billboarding, i.e I rotate the sprite upon the tile in order to always face the projection plane of the camera. If you're interested, you can follow this tutorial.
you can also check this other entry that uses 2.5D: https://alexbarr.itch.io/kayak-ken