It is likely to be similar to how a dozen or more GPS sats can use the same frequency at once without interfering with each other. The outgoing signal from each satellite is modulated with a maximal-length shift register sequence for that specific bird, each sequence being chosen for both minimal autocorrelation with itself and minimal cross-correlation with the others.

I'm not aware of the inner workings of automotive lidar, but I can't imagine building one that didn't work that way.