Do these bolt to the factory rocker panel holes, or do I need to drill?

They bolt to existing factory mounting points — no drilling. Line up the brackets and you're ready to fasten down.

Will these fit under a tonneau cover or bed cover?

Check your cover's side profile first. These mount at the rocker, so most soft or hard tonneau covers clear them, but a low-profile bed cover may interfere. Measure the gap on your setup before committing.

What prep work do the bars need before I bolt them on?

They ship ready to install. If you want to paint them to match or protect the steel, degrease and sand lightly before priming — but bare steel is fine if you're going with that look and don't mind eventual patina.