Does TomTom go work by just your phones GPS or does it have to have a Data connection?

Are the maps placed on your phone and can they be on an SD card?

TomTom GO uses your phones GPS, not data connection.

If the installed SD memory card has more free space than the internal memory of your device, the map and all subsequent content will automatically download to the SD memory card.

If you have Data connection, TomTom Go will indicate the traffic condition, telling you where the traffic jams are. Otherwise you won't see this if you have Data switched off. From what I experienced, it does not use a lot of data though.
The navigation is always on the GPS, not DATA, as you already have your map downloaded.