The VM contains a linux environment (ubuntu) and a ready-to-use development installation of shopware. Its simply only a virtual box image that contains an operation system, a lamp stack and a script to set up a new shopware installation. You can use it for 5.6 too, but this will need some additional configuration.
When changing the NAT to Bridge, you need to adjust the sales channel in the admin of shopware (from 127.0.0.1 to the new ip in your local network) and it should work instantly. The setup contains a command „sw6“ that will set up a new installation from the current github master.