When a user creates an object, that user is by default the owner of that object.
Some create commands have options to specify an object owner, and you can alter an object to specify a different owner. The object owner has full access to the object.
When a user is the owner of a database or schema, that user has full access to all objects within the database or schema, even if he or she did not create those objects. In essence, the database or schema owner becomes the superuser for that database or schema. The admin user has full access to all databases, schemas, and objects.