iPad Magic Keyboard & Sidecar - Solution

So at first, this is actually not a question, this is a solution.

It is well-known that you can't use the Trackpad on your iPad Magic Keyboard while you use the iPad as a second monitor for your Mac using Sidecar. But I'll tell you - you actually can.

Just follow the following steps:

(Make sure they are connected with an USB-C cable, they are connected with the same Wi-FI and you are logged in with the same Apple ID on both devices.)

1. Open System Preferences -> Display -> Add Display -> Mirror or extend to iPad (your iPad should turn into a second monitor in this moment.)
2. Then Click Add Display -> Link Keyboard & Mouse -> iPad
3. Now move the cursor of your Mac to the iPad and Click. (In this moment, Universal Control has connected the devices with each other.)
4. Now straightly move the cursor of your iPad in the direction of the Mac, until the cursor appears on your Mac and click on the Mac's display.
5. Now move the cursor back to your iPad using the iPad's Magic Keyboard Trackpad and Click on the iPad's display.

Now you can use your Magic Keyboard's Trackpad for your Mac in Sidecar.

Hope it helps :D

~ Dago