In France you can fill your tax form online since 1999 https://fr.wikipedia.org/wiki/T%C3%A9l%C3%A9d%C3%A9claration... In the past years banks and companies give tax related information to the tax authority so for lots of people the online tax form is already 100% filled and correct. For the past year tax filling I just had to add to the web form renting income, tax deducible donations, and that was it.
How does France build these systems? Does the government itself have IT development resources or do they contract out to private industry?
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.