More Perfect Union released a video on this a few weeks ago. Worth watching: https://youtu.be/3VJT2JeDCyw

By not pinning apps to every dock position in the Pixel Launcher, it suggests apps based on your usage. It shows a combination of recently opened apps, ones you often use alongside an app you’ve just opened, and ones you usually use at certain times of day. I left the 3 slots open some time ago to see how it works and just left it like that. I kinda like it.

The middle three apps in the dock are dynamic