Understanding of Oracle multi-tenant PDBs

I’m a new user to Oracle. The document states

The CDB root, also called simply the root, is a collection of schemas, schema objects, and nonschema objects to which all PDBs belong.

and

A PDB is a portable collection of schemas, schema objects, and nonschema objects that appears to an Oracle Net client as a non-CDB.

So both CDB and PDB are a collection of schema, schema objects and nonschema objects. How could PDBs belong to CDBs? What does it mean by something like schema object a belong to schema object b?