Can you explain this? maybe it's because of the filter bubble i live in but i almost never see this on whatsapp (then again, maybe i wouldn't know if i had.)

The fact that Facebook walls are visible to everyone, any controversial topic usually gets reaction from both sides. Whatsapp groups are formed based on likemindedness (I am excluding core family and friends). I call these groups echo-pipes which silently form a larger echo chamber.

Are these public groups? How are they discovered?

WhatsApp groups are not discoverable (or publicly listed) unless someone invites you or provides you with a specific invite link with which you can join the group.

As someone who hasn’t used WhatsApp, I am surprised and curious about this. Any other experiences or information would be much appreciated.