Duplicate the volume where macOS is installed and make it bootable
The easiest way to do this is with a commercial tool like Carbon Copy Cloner, which, as you can guess from the name, will do this easily. It is primarily targeted at making backups, but it can definitely do what you want in one step if you have the empty volume ready.
The free way to do this is to make a complete Time Machine backup, install macOS on a new, empty partition, and when setting up the new OS installation, you will have the option to restore from a Time Machine backup, and you restore from the one you just made. This is a lot slower, as it involves making 2 copies of your current drive and the macOS install takes time, but it is free and leaves you with a Time Machine backup.
Note that you do not want to make an exact copy of your current drive the way a low-level utility like dd would. You want the new drive to have its own UUID, for example, so that tools can tell the 2 drives apart. Also, macOS now (Catalina) is actually 2 partitions, a read-only one with System files and a read-write one with User files, and you need to set that up properly. Plus the Recovery Partition is a third partition. You want to use specialized tools that know about and handle these kinds of special needs for creating a bootable macOS drive.