Yes, you are quite correct. Here you can file your taxes either on paper, which means you’ll have to fill in everything yourself, or you can use the website of the tax authority (Belastingdienst). In the latter case the most important stuff is usually already filled in, like your earnings, remaining mortgage & interest paid over it (which gets you a tax deduction), etc, so you’ll only need to check if the prefilled data is correct and add the stuff that the government doesn’t know about.

Bit of a correction on what you said, you not only have to add deductions yourself (some of which are actually already filled in, like the previously mentioned mortgage interest), but also other kinds of taxable income or wealth that couldn’t be filled in in advance, e.g. that bank account in a sunny country that doesn’t share data with our government.

For standard cases you can be done in 30 minutes tops, it is actually a pretty good system, and you indeed immediately get to see either what you have to pay or what are getting back. Although you’ll need to wait for the official message from the Belastingdienst which can take a month or two. But usually that won’t differ from what you were already shown.

Also nice, if you made a mistake you can simply correct it by opening your previous filing digitally, you can adjust a filing up to 5 years ago. Depending on what you adjust do expect to have to deliver some proof, as those adjustments are checked by an employee of the Belastingdienst. The regular filings are checked randomly.