Stage Creation

please make a video tutorial of a level being made from scratch

Yep, I've had the vertical issue before.  It's due to the plane not being in the exact right spot.

Is your default training stage messed up as well (like in video)?  If so, you'll need to reimport that asset.

I don't have the facility to make a video, but it it's pretty straight forward.  The Plane GameObject needs to be at world Y=0.  If lower, you get the vertical issue;  if higher, you get characters falling through the stage.

Also check it's aligned with in Z 0 (the default plane is fairly deep, so you can move it between Z -12 to Z 12 or so and you'll still be OK).

See how you go, I can take some screen shots if you still need help .

Ah, yes.  If you drag into the Scene window, it won't be at world 0.  But if you drag it into the Hierarchy window instead, it will be at its prefab position.  I've made it a habit to drag things into the Hierarchy instead of Scene window, so didn't realise others could be dragging into the Scene window .