I get a letter stating how much I owe or will get reimbursed from the information they have.
If I have additional revenue or tax deductions that aren't listed in the prefilled forms, I can add them and submit them (or have a tax accountant do it).
The letter comes with an envelope that doesn't need postage.
Most people don't have to do anything but simply see money appear on their bank account or pay what they owe by a certain date and that's that.
EDIT: Oh and we can access our taxes online with our national IDs.
So while you can opt to do everything on paper, you or your tax accountant can do your taxes online instead of filling in the forms in case the prefilled forms missed anything. The website will have the same info prefilled as the papers did.
And the taxes website has help sections and information popups to explain pretty much every single tax code imaginable.