Syncronizing process difference between replication and alwayson high availability
Transactional Replication uses the TDS protocol and applies changes using SQL.  The connections are established from the Distributor for Push Subscriptions, and from the Subscriber for Pull Subscriptions.

AlwaysOn Availability groups use TCP/IP connection between the replicas using the Database Mirroring Endpoint.  The connections are established from the instances hosting the Secondary replicas to the instance hosting the Primary replica.  But as the Primary replica can move, every server hosting a replica needs to be able to connect to every other one.