PostgreSQL. Shared_memory and sessions
Yes, shared_buffer is not allocated per-session or per-user. It is shared amoung all sessions, and it is allocated only once.
When parameter is not shared, it is clearly stated, i.e., on page https://www.postgresql.org/docs/9.6/runtime-config-resource.html you may see that on parameter temp_buffers description, it is stated that they are "temporary buffers used by each database session".