When you upgrade to macOS 10.15 Catalina, your boot volume will effectively be split into two. Assuming it’s the standard internal storage, your existing boot volume will be renamed to Macintosh HD – Data, and a new read-only system volume created and given the name Macintosh HD. However, when your Mac starts up in Catalina, you won’t see the Data volume, as it’s hidden inside the System volume, in what Apple refers to as a Volume Group.
I miss the olden days where disk layouts were simple and straightforward. Look at the partition layout of any recent operating system, and you’ll be greeted by several small partitions with specific functions, such as boot manager partitions, restore partitions, and so on. These partitions are hidden, and I’ve always been of the school that if you need to hide something, you probably designed it wrong.
In any event, I understand why this is necessary, but that doesn’t make it any less hacky and messy.
So this is a similar arrangement to what users of other Unices have been doing for years — having a root volume or partition, and a data volume, except that there are now programs as well as data (i.e. not just the /home tree) in the data volume? I don’t mind this in itself as that’s how my Linux system is partitioned, but yet again, Apple messes around with our disks during an upgrade without our consent (same happened with the ‘upgrade’ from HFS+ to AFS which meant we could not access our Apple data from within Linux).