In a system with several microservices, how does an event published by one service end up reaching a handler running inside a completely different service?

Every microservice in the system connects to the same shared Axon Server instance over gRPC, rather than connecting directly to each other. When a service publishes an event, it sends it to Axon Server, which durably stores it in the event store and then routes it outward to every handler registered to receive events of that type — regardless of which service that handler happens to be running in. The publishing service never needs to know which other services exist, where they're deployed, or who's listening; it only needs to know about Axon Server. This is what lets microservices stay decoupled from each other at the network level while still reacting to each other's events, and it's also what makes adding a brand-new consumer of an existing event stream possible without changing the publishing service at all.