Install backup of my old Mac on my new one I got a new MacBook Pro with Catalina on it. I made a APFS formatted Copy of my old Mac and want to install it on a separate Partition on my new MacBook Pro so that when I start the new MacBook I can switch between the new one and the old one but I can't figure out how to do that.
You could use the Disk Utility to create a new APFS volume in your current APFS container. Next, download macOS Catalina from Apple App Store which will appear as the Install macOS Catalina application. Run this application and install Catalina in the new volume. Boot to the new volume and run the Migration Assistant application to transfer applications and data from the old Mac. This transfer is in the form of a copy. In other words, nothing is removed from the old Mac.