How do online subsystems work with cloud hosting platforms like Gamelift or Playfab

Say I’m using the Steam OSS for creating sessions, handling friends, etc. But I decide instead of doing a listen server setup I want to move to dedicated servers hosted on the cloud. I’ve looked into PlayFab and Gamelift and they both offer session systems and matchmaking. My question is: how would a hosting service work with an online subsystem? Do you use them together? Or ditch the OSS? I’ve just starting learning about dedicated servers/hosting, sorry if this isn’t worded quite right.