How is the service map generated?

It is generated based on the trace data you send to SigNoz. We run the ClickHouse materialized views on the ingested data to build the RED metrics and show the service dependency metrics on it https://github.com/SigNoz/signoz-otel-collector/blob/main/migrationmanager/migrators/traces/migrations/000022_dependency_graph_minutes_v2.up.sql