No, you don't learn that at uni, but you potentially gets exposed to one or two databases (unfortunately often something proprietary that should be classified as cruelty at that point, like Sybase).

You rarely learn to select the right database in general though your whole career. Normally you're in one of two situations: you don't know of any special requirements you have and go with an SQL database you like, or you know your special requirement and know what to look for.

I wouldn't start with dynamodb, because it's rather specific and has some limitations you may not expect. If it's your first backend project, use something general-purpose and popular so you can find lots of resources around it. Most likely postgres, MySQL, or sqlite (if you don't need a server).