What's /System/Volumes/Data?
Catalina introduces a new file system layout. Where Mojave and earlier had one filesystem that combines the system and user data, Catalina has a read-only system volume and a read-write user volume interleaved on a folder by folder basis using firmlinks.
The easy way to move forward is just save your additional files to /usr/local and other traditional places where Apple expects user modifications to their default system to be saved.

Check out What's New in Apple File Systems from #WWDC19
PDF summary of the above video presentation

Some of the implementation is quite normal for Unix/Linux like sparse files not being allocated and copy on write and cloning of an entire file system / snapshots. Other items like Firmlinks that act as “wormholes” between two containers / filesystems to present an unified file tree, System Integrity Protection and APFS specific features are quite new still to everyone.
You can see this better with df or diskutil apfs list command line tools than the Disk Utility view.