MongoDB 1,815 ships in three shapes, and the differences are licensing and operations, not query language: everything in The Document Model and BSON through Transactions and Streams behaves identically in all three. Community Server is free under the SSPL. Enterprise Advanced is commercial and adds LDAP and Kerberos authentication, auditing, an in-memory engine and Ops Manager. Atlas 1,815 is the managed service, free from the M0 tier up, and the only place MongoDB Search and Vector Search run without an Enterprise license.
The license matters more than it looks. Community moved from AGPL v3 to the Server Side Public License for every release after 16 October 2018. Section 13 says that if you offer MongoDB's functionality as a service to third parties, you must publish the source of everything you use to run that service. Building an application on MongoDB is unaffected; reselling MongoDB as a hosted database is the target. The OSI has not approved the SSPL, which is why Debian 319 and Fedora 1,480 dropped MongoDB from their repositories.
Version numbers read X.Y.Z. Major releases (7.0, 8.0, 9.0) arrive about every two years with five years of support, and are what you run in production. Minor releases (8.2, 8.3), introduced with 8.2, land as needed inside a major cycle and stop receiving patches once the next minor ships. You cannot skip a minor — 8.1 to 8.3 goes through 8.2 — and each step needs new binaries plus a raised featureCompatibilityVersion, which keeps a server writing data an older binary could read.