Here's how. With Sidecar active, select the Sidecar icon in your Mac's task bar. It looks like two offset rectangles. Choose Display Preferences at the bottom of the menu. In the window that opens, you can drag either the Mac or iPad screen to align them in the way that works best for you. You can put the iPad screen to the left, right, above
Sidecar requires Mac hardware support for HEVC/H.265 video encoding, even for a machine with a discrete GPU (which it does not use), and without a T2 co-processor. A 2016 MacBook Pro or later is needed because of two factors. Intel's Quick Sync needs Skylake or later processors for the above encoding issue. Machines with a T2 chip will do the hQAfPEI.