How DD4T application gets the content data from the broker database? odata service or broker api ?
DD4T (both Java and .Net) uses the standard Broker APIs to fetch data from the Broker database and thus requires all the content to be published to the database.
It is not possible to use a file system deployer, since the web application is setup to fetch all content from the Broker database.
There was an OData Page provider created for DD4T once, but when SDL Web 8 introduced microservices, the benefit of that OData provider fell away.