Technically your non plant leg/foot or arms or pretty mcuh anything can be outside the tee area, so long as they arent touching the non tee surface.
Yes, as I said, every supporting point must be within the tee area. If you're only using one leg to support yourself, then that's all that matters. If you're standing on both legs, then both feet must be in the teeing area.
This differs from a fairway shot where only one supporting point has to be on the lie.