They would go in the sections for 401ks and IRAs just like they do on the paper forms. The online form will have the same way to enter the additional deductions.
They should (and do?) have the same information your employer, bank, or brokerage files. i.e. the same forms you use to fill out your taxes now. They know what you contributed to you 401k and your other retirement accounts.
Solo 401ks/IRA also wouldn’t be something they know about until you file if I understand correctly. Guess you could that expenses?
They would go in the sections for 401ks and IRAs just like they do on the paper forms. The online form will have the same way to enter the additional deductions.
They should (and do?) have the same information your employer, bank, or brokerage files. i.e. the same forms you use to fill out your taxes now. They know what you contributed to you 401k and your other retirement accounts.