Whatsapp Messenger like realtime/session oriented apps failover/high availability

Apologies if this is not the right forum for this kind of question. I was able to find high-avaibility tag. I can port this question to meta if needed.

I am curious on how Whatapp Messenger acheives high-avaibility and auto/manual failover. I was able to get some info from here and here. These articles are written back in 2012/2014. Looking for more recent info. According to these articles, WhatsApp uses custom XMPP/Jabber protocol and have been recently migrated to custom TCP protocol. I could not verify it on whatsapp Blog.

What is the protocol used in Whatsapp? How does it manages high-availability/failover? Is there a LB/cluster that can be failed over to port 1M active connections?