In March or April, everyone gets sent their pre-filled tax form for the previous year. You can change it as needed, for example adding income from a side hustle, or making deductions -- though those are fewer and less complicated than the US from what I understand. Many common deductions (e.g. for mortgage interest) will be pre-filled too.
Most people will not need to change anything, and so can approve the pre-filled form through text message or digital signature (or phone robot for old people iirc). If you do need to make changes, it can be (but doesn't have to be) done online without too much trouble. Don't get me wrong, it's not always trivial, but it seems significantly more pleasant than what I hear about the US system. And, again, the vast majority of people just have to send a text message some time in March or April.