General Questions SFB on Prem, SFB online and Teams.

Hey Guys, 

Sorry for these basic questions here. But i may be starting a new job which might require that i provide support for SFB On-Prem/Online, Teams etc. So just some general questions on intergration/interoperability

1) If a user is using SFB On-Prem are they able to connect and chat with Teams users? Or is it client to client? I realize various operating modes come into play here but i just wanted to get an idea as to how it worked. 

2) If a user has Teams only, can they communicate with SFB on prem users? SFB Online users? 

3) Is hybrid co-existence mode required to be setup between SFB On-Prem and SFB online if you want to use a move-csuser cmdlet to migrate a user? 

4) When you migrate a Skype for Bus on prem user to the cloud is all conversation history migrated as well? Or is that just stored in outlook? 

5) Can you have a on-prem mailbox and the same user migrated to SFB online? 

6) Can all users (once hybrid is configured) communicate with each other? meaning can a user homed on prem communicate with a SFB online user and vice-versa? it looks like they can i but i just wanted to be user: https://docs.microsoft.com/en-us/skypeforbusiness/hybrid/plan-hybrid-connectivity

Thanks and sorry for the basic questions. 

Robert