The code that defines how the fact graph works is here: https://github.com/IRS-Public/direct-file/tree/main/direct-f...

but the actual tax definitions that deal with facts and derived calculation are here: https://github.com/IRS-Public/direct-file/tree/main/direct-f...

See for example the standard deduction and tax rate calculations https://github.com/IRS-Public/direct-file/blob/main/direct-f... https://github.com/IRS-Public/direct-file/blob/main/direct-f...

I imagine these are based on the MeF (Modernized e-File) schemas because the system needs to transform the input data into XML MeF schemas to submit electronically to the MeF system (See https://www.irs.gov/e-file-providers/modernized-e-file-mef-s...)