what OS ships with 2020 imac

The very latest version, 10.15.6, which was released on July 15, before the new machines were announced. Check with the developer(s) of your music software to make it is compatible. You cannot easily downgrade a new Mac to a version of macOS earlier than what it shipped with.

You cannot easily downgrade a new Mac to a version of macOS earlier than what it shipped with.

Just for clarification: You cannot at all downgrade a Mac to any version older than what it came with. That means if you buy an existing Mac now which comes with 10.15.4 (as an example), you cannot even boot it from 10.15.2 or .3.

Also, you didn't say what Mac and OS you are replacing. To elaborate a bit, if you are not already running Catalina, you will have to deal with any old 32 bit software apps you still have as they will not run on the new computer. Some may be updatable to 64 bit operation and some will have to be upgraded at a cost to you. So, as you imply you are doing, planning ahead is a good thing.