Not familiar with belgium but in Australia they don’t. It’s linked to your payroll so they know if you paid more or less that it was due, plus they have linked your bank accounts so they know if you had any interest to pay taxes on. Something similar for investments accounts too.
If you want to claim any deductions (ie. You work from home and have home office expenses) you punch them in yourself
In Belgium they also fill in the expenses automatically.
The only thing you need to do is check if anything is missing, and just not do anything if everything is correct. Which it usually is.
How do they know what your expenses are?
Not familiar with belgium but in Australia they don’t. It’s linked to your payroll so they know if you paid more or less that it was due, plus they have linked your bank accounts so they know if you had any interest to pay taxes on. Something similar for investments accounts too.
If you want to claim any deductions (ie. You work from home and have home office expenses) you punch them in yourself
How can it fill expenses automatically? How can they know if something I buy is for work or leasure?