Overlay PenWhat Sidecar does
Sidecar is built into macOS and iPadOS. It extends or mirrors your Mac on the iPad, wired or wireless within about 10 metres, and Apple Pencil can point, click and draw in Mac apps that support it. It is free and excellent for that job. It needs a Mac signed in to the same Apple Account.
Why it is not a drawing layer
Sidecar shows Mac windows. To draw over your camera or slides you still need an annotation app on the Mac, and then the ink sits on your screen rather than in a clean, transparent layer your streaming software can place on top of the picture. Duet Display and Luna Display work the same way, and also support Windows.
How Overlay Pen is different
Overlay Pen is not a second screen. It is a pen for your video. You draw inside a 16:9 capture area that matches the shot, and only the ink goes to OBS Studio or Ecamm Live as a transparent web overlay, sharp up to 4K. From there it reaches Zoom, Teams and Google Meet. It works with OBS on Windows and Linux too.
Questions
Can I draw on my screen with Sidecar alone?
Only inside Mac apps that accept Apple Pencil input. To draw over anything on screen you need an extra annotation app.
Does Sidecar work with Windows?
No. Sidecar needs a Mac. Duet Display also works with Windows.
Do I need a Mac for Overlay Pen?
No. The web overlay works in OBS Studio on Mac, Windows and Linux. Ecamm Live is Mac only.
Can I use Sidecar and Overlay Pen together?
They do different jobs. Most people use Overlay Pen on the iPad while presenting and keep the Mac on its own screen.