how are synthethised historical instrument like the 1849 Erard?

Welcome to the forum, yussef961! My understanding is that the historical instruments are prepared in a similar way to the modern ones, using the instrument in its existing state at the time of processing. All the historical pianos and harpsichords are modelled from real instruments in museums or private collections. After the initial preparation, the sound is then modified as necessary to create a 'virtual restoration' - aiming to capture the sound as it might have been when the instrument was first made. This process can be heard to good effect in the NY Steinway Square: the 'Original' preset is the piano with 'authentic owner settings' and the other presets are the modified versions.

It's great to meet another enthusiast for the historical pianos - they brought me to Pianoteq, too!

Yes, I would very much like to hear original recordings of the real instruments, unfortunately Modartt don't share them if they have any (it would be great if they did - even if only a few brief samples).
No French section for the forum, but there are quite a few French speakers who are regular forum members (Gaston, for example), and they sometimes post phrases in French.