AppleConnect is a small Swift wrapper around a Network TCP stream on the local network (using Bonjour for discovery). While the transport is bidirectional, the API is designed around a "service" provided by a single server and connected to by potentially many clients.
Setup for servers differs a little bit from clients. Once a connection is established, the channel is identical from both ends. All connections are encrypted using TLS-PSK derived from a shared key of your choosing.
Important
For security, you should generate the shared key using cryptographically appropriate random data. Sharing this key should be done out-of-band and is out of scope for AppleConnect. For user-facing applications, one way you might do this is by generating a code on one device and asking the user to confirm it on the second one.
A typical server should advertise its availability using Connection.advertise(forServiceType:name:key:). Attempts by clients to connect will show up as NWConnection objects, which you can pass to Connection.init(connection:) to complete the connection process.
A client should browse for servers it wants to connect to. Connection.endpoints(forServiceType:) will asynchronously stream a list of available NWEndpoints, and once you've found an endpoint that you'd like to connect to, call Connection.init(endpoint:key:) to establish the connection using the shared encryption key.
Both clients and servers can send data to each other using Connection.send(data:), and receive data by watching Connection.data.