mongoc_connection_pooling(3) on Linux
The text e8406245bc9e, as Debian 11 (Bullseye) shipped it.
Connection Pooling The MongoDB C driver has two connection modes: single-threaded and pooled. Single-threaded mode is optimized for embedding the driver within languages like PHP. Multi-threaded programs should use pooled mode: this mode minimizes the total connection count, and in pooled mode background threads monitor the MongoDB server topology, so the program need not block to scan it.