In Flux's PCB layout mode, the auto-layout option is accessible via the "Auto Layout" button located in the lower toolbar. Just switch to the PCB tab and look toward the bottom right of the interface—you’ll see the Auto Layout button there. Clicking it will initiate the automatic routing of your board based on the connected nets.
Yes, Flux provides a preview mode when you initiate the auto-layout. Once you click the Auto Layout button, the tool generates a temporary proposed routing overlay on your PCB layout. This preview lets you review the changes—ensuring that the routing, trace widths, and overall layout meet your design requirements—before you finalize the modifications by clicking the Apply Changes button.