https://www.irs.gov/e-file-providers/free-file-fillable-form...
As for "having most of your tax information", they don't. They know your reported income. You see that on your W2s/1099s/etc. What they don't know is whether or not you had a kid this year, or whether you lost a kid this year, whether you got married or divorced, if your spouse is claiming the kids this year or not, the number or amount of your charitable contributions, whether you have deductible mileage expenses, or a million other things.