Each process runs in a separate VM but on the same physical host. Useful when you’d like to run, say, multiple microservices that need to be as close to each other as possible, but have their own scaling characteristics. See: Preview: multi process apps (get your workers here!)
Multi-process Fly apps are in beta, afaik, and it is unclear (to me) how autoscaling / vm scaling works.
One could also run multiple processes in the same Fly app (single VM), ref: Running Multiple Processes Inside A Fly.io App · Fly Docs
Or, one could deploy multiple apps (from the same codebase) as different Fly apps (separate VMs across hosts): Monorepo and Multi-Environment Deployments · Fly Docs