For the tax system it was developped with lots of free software, and support purchased to French IT companies. It was hosted on a big (for the time) linux server farm.

Here are a few interviews and slides by the director of this project, unfortunately in French (didn't check automated translations, might work):

https://www.toolinux.com/article/jean-marie-lapeyre-copernic...

http://www.supelec-rennes.fr/sic/JOURNEES/05_10_20/lapeyre.p...

The free software community in France was very happy to see one major ministry doing this kind of projects at the time.