The default type of index created for an In-Memory Neighbor Graph vector index is Hierarchical Navigable Small World.

• Understand Hierarchical Navigable Small World Indexes
  Use these examples to understand how to create HNSW indexes for vector approximate similarity searches.
• Understand Transaction Support for Tables with HNSW Indexes
  Hierarchical Navigable Small World (HNSW) index graphs are static memory-only structures. Transaction maintenance on tables with HNSW indexes is done by using two main structures: private journal and shared journal.
• Understand HNSW Index Population Mechanisms in Oracle RAC or Single Instance
  Learn how Hierarchical Navigable Small World (HNSW) indexes are populated during index creation, index repopulation, or instance startup in an Oracle Real Application Clusters (Oracle RAC) or a non-RAC environment.