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.