When I worked at Discord, we used BEAM hot code loading pretty extensively, built a bunch of tooling around it to apply and track hot-patches to nodes (which in turn could update the code on >100M processes in the system.) It allowed us to deploy hot-fixes in minutes (full tilt deploy could complete in a matter of seconds) to our stateful real-time system, rather than the usual ~hour long deploy cycle. We generally only used it for "emergency" updates though.
The tooling would let us patch multiple modules at a time, which basically wrapped `:rpc.call/4` and `Code.eval_string/1` to propagate the update across the cluster, which is to say, the hot-patch was entirely deployed over erlang's built-in distribution.
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