Understanding animations

[Estolle00]

Hello,
I'm having a bit of trouble figuring out how the sprite animations work in SWN. I've played around with Godot before, but I used a different technique for animating. It looks like all of the animations are strictly determined by the sprite placement on the spritesheet (for example, every character's run animation occurs on the second row).
My difficulty is coming from not figuring out where these pre-defined sprite "positions" are...defined. What I'm trying to do is add a new animation for Sonic (not overwriting an old one), but I can't figure out how to create an animation that is pointing to those specific sprites in the same way that the others are. Hopefully this makes sense.