Two things going on here:
First, Oracle defines "database" differently than other "databases" like MySQL. When Oracle talks about a database, they mean the physical instance of the software, data files, and processes that run on on the server. Each database instance may house hundreds or thousands of individual users/schemas, each with their own tables and other objects. It is possible to run multiple database instances on a single server - even using different versions of the software simultaneously. Operating multiple instances/databases - usually but not always of the same version - on a single server is quite common, actually.
Most other database engines equate "database" with a single schema of objects, and often only support one installation of the software on an individual server, though they may house hundreds or thousands of users/schemas.
Second, Oracle's pluggable databases (introduced in Oracle 12c) are a virtualization of the database instance (the running processes, data files, etc.) that allows them to share some common, system-level resources. A container database (CDB) manages those system level resources, and may host many pluggable databases (PDB). A single server may host multiple container databases of different software versions, each with their own pluggable databases. Each pluggable database may contain hundreds or thousands of users/schemas. The pluggable database architecture allows for consolidation of compute and storage resources: a higher density architecture that makes more efficient use of both on-premise and cloud hardware.