Most of it is prefilled in my french declaration.
I had to check a box saying that I don't own a TV (so I don't have to pay that tax)
I also had to connect to my home cleaning service website in order to know exactly how much I paid them in 2017.
That's pretty much it, everything else was already filled.
Since my employer has declared that I worked for them (I mean .. that's the case for most people except black market), the FISC (french IRS) already knows how much I made that year.
If you are freelancing, or another regime different from full time job in a company, it might be more complicated.
but for 90% of the people, you connect to the gov website, approve your taxes and that's it.
US has federal and state taxes. Both are obscure. AFAIK, there is no government website to do your taxes. No prefilling either of course.
So I had to google a lot, look for my w2, etc.
I used hr block since it looked easier than figuring out everything myself.
It was still more of an headache than my french taxes and I don't think that it was only because of my expat status.