Once microservices exceed five, the biggest cost in a system is no longer code but coupling. A service directly calling B and B calling C creates a chain that seems simple at first, but B's failure propagates to A, and every time a new service D is added, both sides' code must be modified. The pattern that solves this is Event-Driven Architecture (EDA). Services publish events, and other services subscribe to them. Publishers don't know who's listening, and subscribers don't know who sent it.
AWS has multiple EDA tools (SNS, SQS, Kinesis, EventBridge), but the biggest shift in the past 5 years is that EventBridge became the de facto standard router