A catalog is a folder.
The catalog name is the folder name.
As you can see, both catalogs (with different names) are stored in the same default catalog location.
Each organizer has its own catalog version. PSE2020 can't open catalogs from PSE2019, it has to convert the old catalog to its own format. Then the converted catalog inherits the name of the old catalog, and the old catalog is renamed with a -1 suffix. The old organizer recognizes automatically this renamed catalog folder.
So, how do you know which version is a given catalog.
In the explorer, you look at the contents of the folder.
PSE2019 contains a file named 'catalog.pse17db
PSE2020 contains a file named 'catalog.pse18db
If a catalog file has a -1 suffix, that means it has been converted
If a catalog file has a -1-1 suffix, that means it has been converted twice.
You can convert a catalog several times. It is shown in the available catalog list when you tick the checkbox 'show already converted files'.
The normal procedure when you install PSE2020 is that you receive a prompt to convert the old catalog. It seems you have missed that opportunity. Not a big deal, you can convert manually afterwards, even several times. If there has been no catalog conversion (may take some time), you must start it yourself from the catalog manager.
You have the choice to keep the catalog folder either in their default location, or anywhere else in your computer. You only need to move the catalog folder from the explorer. For instance you can create a catalog folders tree with subfolders for a given PSE version (see the 'catalog.pseXXdb' criteria above).
To open a catalog from the 'custom' location, you can either navigate in the catalog manager, or simply double click on the catalog.pseXXdb file in the folder, which opens the organizer with the chosen catalog.