The sensor has special pixels which can detect phase differences in incoming light. Camera uses these pixels (aka embedded detectors on the image sensor) to be able to focus where you want. This is same from phones to professional cameras.
(DSLR cameras have independent phase detection arrays. This is why the mirror has a small mirror behind. To illuminate that section).
Lenses are tuned. They know how distant the thing they are focusing on, also photos' EXIF generally carries the focus point information. Side note: Apple Aperture used to be able to show where you focused with that info. Still no app does this. I'm still mad that we don't have Aperture anymore. Anyway...
So, if you collect this phasing information alongside the photo which you're taking, you are capturing the depth map of the photo. Its resolution will be lower, but not lower enough to be useless.
For example, Sony (and most probably all other big camera manufacturers) cameras use this phasing data throughout sensor in real-time for following moving subjects to keep the focus on them by predicting where they are going.
So, there's no "in theory". The phasing data is the depth map. Otherwise your camera can't focus on anything. It's there to focus, and it's done by reading the phasing data and knowing where to go by how much.
All fast AF systems are PDAF. When light is too low, then it's CDAF, which does way slower, without using any phase/depth data.
[0]: https://en.wikipedia.org/wiki/Autofocus#Phase_detection
Edit: Meaning clarified, bugs squashed, mirrors polished.