You guess with a calculator and detailed instructions. Educated guess.
Here in Sweden they just come finalised. There may be occasions where you’ll need to make changes, but that’s literally never happened to me. “Doing taxes” involves me logging onto a website, skimming over a page or so, and then digitally signing it using my phone. Takes less than five minutes.
Here in the US, we have companies who offer their services to prepare taxes for you, and they spend an unreasonable sum lobbying the government specifically to not do what you lucky folks have been getting all along. Just can’t have the US without unfettered capitalism ruining things for average folk. :(
You do engineering with calculator and detailed instructions, educated guess. You do mechanics with detailed insturctions. You do cooking with detailed instructions. You do rocket science with calculator and detailed insturctions. Just be built diffrent ladies! It’s not THAT hard or an entire predetory industry, JEEZE FELLERS! /s
Are you suggesting that putting a number from box A into box B, and subtracting box C from box D, is as hard as rocket science???