APFS and El Capitan

After replacing a failing hard drive in my Mac Mini (late 2012) with an ssd drive I decided to update from El Capitan to Catalina. Baaaadddddd mistake. Many programs I depend on wont work.... I tried to restore from backup and go back to El Capitan and got the message about HFS+ not being able to restore to APFS drive. While trying to erase the SSD drive with intentions of changing it to HFS+ file system there were no options to do so. All options were APFS. There was a button in the lower left corner that said something about pooling or grouping and I pressed it out of desperation and the structure of the drive changed from 2 volumes to only one and the drive erased. I pressed erase again to see if I could format and either get my other structure back or format to the HFS+. Still no option for anything except APFS. I went ahead and formatted again just for grins... HOWEVER, now I was able to restore from my HFS+ backup onto my new hard drive which went with no problems. I have El Capitan running on my new SSD drive. I am attaching drive info for you to see.

My question... Is this OK??? What was that button that I probably should not have pushed but did any way? I don't usually go pressing things until I know what they are. I searched the web and found no info about this mysterious button that seemed to resolve my issue. Any help/advise would be appreciated. thanx, SparkyzFan