With your French D visa + transit Schengen, you can enter the French territory (on your way to France, transit through any other Schengen country), then, once in France, go to a "préfecture" to request a "carte de séjour". The "préfecture" will give you first a "récépissé" of your request while it studies your file.
Once you have a "carte de séjour", that document allows you to travel freely within the Schengen area.
However, in the meantime, with only your "récépissé", you are not allowed to travel within the Schengen area. If you wish to travel, you have to request a visa for the Schengen country you want to visit.