SQL 2016 always On design

Hello, I need your help to understand how the distributed AG works betwenn data centers. here is what I need to impliment:

2 datacenters without a L2 network link

4 instances database (liveDB : RW all the 24/7)

2 VMs for each instance per site AG on each site DAG between site

my questions:

should I have a didicated network between the 2 sites to handle the hearthbeat: (L2 or L3 ?)

how to calculated the needed bandwith : just log transaction?

Thanks