Since there are two partitions this step will be done twice.

Select the new (currently external) SSD that has catalina on it. Select the first partition. I've called my disk MacGuyver

Then Click restore and choose the corresponding partition on your existing Catalina disk still installed in the computer

Once that is complete Repeat the recovery process for the second partition, this may have the word - Data on the end of the named disk.

This step is critical. The goal is to recover your data from the old drive to the new drive. Catalina uses two partitions, so the step must be done twice. This can take a while...