Neutron is indeed a framework for managing virtual networks. How those virtual networks are implemented by the underlying infrastructure is determined by plugins, for example the core plugin for the core resources network, subnet and port, the L3 plugin for routers, the DHCP plugin for (you guessed it!) etc.

Neutron contains its own reference plugins. The reference for the core plugin is ML2, and the two reference mechanism drivers are LinuxBridge and OVS.

As far as I understand the meaning of "default", it means "will be used if nothing else is configured". If you don't configure a core plugin, Neutron won't work (I guess the neutron server would issue an error message and stop), so that in my mind, ML2 can't be considered "default". But perhaps that's just a semantic game.