The query router, also known as mongos process, acts as the interface and entry point to our MongoDB cluster. Applications connect to it instead of connecting to the underlying shards and replica sets; mongos executes queries, gathers results, and passes them to our application.
mongos doesn't hold any persistent state and is typically low on system resources.
mongos is typically hosted in the same instance as the application server.
It is acting as a proxy for requests. When a query comes in, mongos will examine and decide which shards need to execute the query and establish a cursor in each one of them.