Upgrading to El Capitan on iMac 2008 - two Apple Disk images?

Hi, please advise...I'm upgrading my old iMac 2008 from Lion to El Capitan because it cannot install the latest Catalina (I tried with the patch but that didn't work). Nothing on the machine, it's a clean install.

Installed OSX Lion, the did all the software updates, no probs, working fine. Then downloaded ElCap from apple, installed the package to check machine can take installion (it said yes), then created external usb boot disk of El Capitan installer to do the upgrade. Installation didn't work completely, but it's further than I got before...error on HD with First Aid...so I erased the HD and it fixed some boot something or other. All drives now get a tick on checking with First Aid.

But I seem to have two Apple Disk images...is this correct? - see Terminal diskutil list pic and also pics of disk utility. Base system is 2GB which I guess was Lion and next one is El Capitan at 6GB....my question

is should I delete the Base OSX system or not? Thanks!