That's not (entirely) true. You fill in the tax forms online on the website of the tax office, but only the known income, bank accounts, and mortgage have been

That's not (entirely) true. You fill in the tax forms online on the website of the tax office, but only the known income, bank accounts, and mortgage have been filled in already. If you're an independent contractor, own a company, have stock, pension plans outside the normal pension funds, donated to charity, etc., there's still a lot to be filled in.