The beam is split and re-emitted in multiple points. By controlling the optical length (refractive index, or just the length of the waveguide by using optical junctions) of the path that leads to each emitter, the phase can be adjusted.

In practice, this can be done with phase change materials (heat/cool materials to change their index), or micro ring resonators (to divert light from one wave guide to another).

The beam then self-interferes, and the resulting interference pattern (constructive/destructive depending on the direction) are used to modulate the beam orientation.

You are right that a single source is needed, though I imagine that you can also use a laser source and shine it at another "pumped" material to have it emit more coherent light.

I've been thinking about possible use-cases for this technology besides LIDAR,. Point to point laser communication could be an interesting application: satellite-to-satellite communication, or drone-to-drone in high-EMI settings (battlefield with jammers). This would make mounting laser designators on small drones a lot easier. Here you go, free startup ideas ;)

In principle, as the sibling comment says, you could measure just the phase difference on the receiver end. The trick is that it's much harder for light frequencies than radar. I'm non even sure we can measure the phase etc of a light beam, and if we could, the Nyquist frequency is incredibly high - 2x frequency takes us to PHz frequencies.

There might be something cute you can do with interference patterns but no idea about that. We do sort of similar things with astronomic observations.

A phased array is an antenna composed of multiple smaller antennas within the same plane that can constructively/destructively aim its radio beam within any direction it is facing. I'm no radio engineer but I think it works via an interference pattern being strongest in the direction you want the beam aimed. This is mostly used in radar arrays though I suppose it could work with light too since it is also a wave.

I think about it like a series of waves in a pool. One end has wave generators (the lasers) spaced appropriately such that resulting waves hitting the other end interfere just right and create a unified wavefront (same phase, amplitude, frequency).

NB: just my layman's understanding

Not an expert, but main challenges with laser coherency are present when shaping the output using multiple transmitters.

For lidar you transmit a pulse from a single source and receive its reflection at multiple points. Mentioning phased array with lidar almost always means receiving.