It depends on which technologies i'm working on, for example if I'm working on JavaScript, then I usually use MongoDB because it integrates better with JavaScript especially as MEAN Stack.
If i wish to use SchemaLess database that uses tables to store data then you can use DynamoDB and if you can use SchemaFree database that uses Documents to store data then you can use MongoDB.
If you wish to learn MongoDB you can do this end to end certification course on MongoDB.