This is the layout of the previous Mojave startup volume:
This is a layout of the original Catalina startup volume:
This is the layout of the latest version Catalina startup volume:
This latest version provides much more protection of your data as the data is separated to its own volume, keeping the system files on their own seperated volume.
Here is just a peak at what the latest version of Catalina upgrade has to accomplish:
For the Catalina installer to complete successfuly, it has to do the following:
rename your existing startup volume with ” – Data” appended;
remove all existing system folders and files from that new Data volume;
change the folder layout on the Data volume;
create a new System volume, and install the new system there;
set the volume types of the System and Data volumes, and convert them into an APFS Volume Group;
create new firmlinks to connect the two volumes.
The installer also has to update the firmware in your Mac, and replace the contents of the Recovery volume.
With all that and more, this is why it is so important to make sure the existing startup hard drive and system is in tip top shape, before upgrading. Also, make sure you have a backup, just in case.
Is it worth upgrading, You betcha...