Streaming replication in postgres question
This is not an inherent feature of PostgreSQL.  Whether your application will connect to the replica (or even knows that a replica exists in the first place) is up to your application, framework, library, connection pooler, or networking.  And their respective configurations.

If it does connect to the replica, that connection will be read only, unless you do something to promote the replica.  Autopromotion is not an inherent feature of PostgreSQL, you would have had to install and configure something to make it happen.