In order to use a database to load original data for synthesization with MOSTLY AI, you only need to create a connector to your database and enter your authentication details. Currently, MOSTLY AI allows you to connect to databases like BigQuery, MS SQL, MySQL, Oracle and PostgreSQL, etc.
In order to connect to a local database that's hosted on your machine, it will be necessary to expose it to the Internet. Data destinations on a local machine use localhost as the endpoint, which is the default name of the computer you are working on. If you want to expose your localhost to the Internet in order to accept connections, it would be necessary to use a tool like ngrok to allow external connections. If your goal is to connect a local database to MOSTLY AI, we have a tutorial on how to create a data destination on your local machine.