I dont think theyre thinking about profits here, but abouttheirr users
Other data replication technologies worth looking in to: GlusterFs, Ceph.
Dependent on your db's they should offer replication out of the box.
You can also implement a load balancer, such as HAProxy or Nginx, to distribute incoming network traffic across multiple VMs
If you plan on using this in a production environment, I'd bring in a consultant.
However, I've heard of people in the home-lab sphere use things like heartbeat and drdb. The more nodes the merrier as if you lose connection between the two you'll have a bad time.
I purchased (havent deployed) lemmy.club yesterday and I'm going to look in to deploying via kubernetes this weekend. We'll need some instances that can horizontally scale so that we can absorb as many users as we can from the APIcalypse.
I'm slightly worried about the costs, but I'm loving the platform so far and am willing to do as much as I can!