The complement of onto can be a prospect or anything that leads away.
The preposition on can refer not only to to the placement of something (She put the hat on her head); it can refer to the direction in which one (the one being spoken about) is facing. Hence onward and They marched on. And direction always has two elements, relative orientation and continuation.
to has the sense movement in a direction.
He went to the window and looked out.
Combined, on + to indicates something with extension-in-space there in the direction in which one is facing.
The doors opened onto a large terrace.
A person standing at that place, having opened the doors, will find a terrace there in front of them.
We would not say (except in a jest or ironic use)
The doors opened onto a small broom closet.
A broom closet lacks real extension-in-space-in-the-direction-one-is-facing (not to mention the fact that the closet door doesn't swing out into the closet area).
The door opened onto an alley which eventually opened onto a wide thoroughfare.