There are details and plans published in a several French books and magazines.
A good book or series of books is the 'Autorails de France' published by La Vie du Rail.
They're up to volume 5 now. Not cheap but invaluable reference for all French railcars.
Unfortunately most of my books and mags are packed away now.
The Billard 75(CV) were part of a small batch of 5 delivered in 1947/1948.
3 for the private Mamers St Calais line in the Sarthe no.s 901-903 and 2 for the CFD Saone et Loire.
The requirement was for a lightweight twin axle machine weighing no more than 8 tons.
They were powered by a small 4 cyl. Panhard engine transverse mounted with the radiator to the side. This peaked at 2000 rpm.
Overal length was just under 12 metres. Seating capacity a mere 32. They were modified later on so that they could be 'coupled' together.
They were not actually capable of proper MU operation as far as I know and they only had air assisted shoe braking.
In fact many railcars were just coupled but driven indepedently with different drivers using a system of signals and commands often by hand.
There was a whole wave of lightweight railcars at this time resulting in some standard compatible designs from different manufactures, starting with FNC agreement (Féderation Nationale Cheminots) and culminating in 'les autorails unifiés'
of which a number still exist today.
It would be fair to say that the French were probably a good ten years ahead of the UK in terms of railcar/railbus design and technology.
Renault for example were building high capacity, high speed semi-streamlined machines with V12 turbocharged engines before WW2 even for metre gauge.