I don't have much (any) experience with UNet, so I may mislead you unintentionally, but:
OnStartClient seems to match up with OnPlayerJoin. OnPlayerJoin is specifically for when other clients join a channel you're in. OnJoinChannel is for when you join a channel.
ClientRpc and
Command seem to be RFCs but differ in who sends / receives it. With TNet, if you want an RFC to execute on the host only, you simply check if TNManager.isHosting returns true. If not, return. Furthermore, the sender has some control over who receives the RFC. Target.Others, Target.Host, Target.All, and others.
Don't forget the tutorials stickied at the top of this forum. They, along with the examples you're already going through, should ease the transition a bit
