The "collapse" of the government in a parliamentary system means the government no longer commands the confidence of a majority of the (typically) lower house, in this case the House of Representatives. Nothing immediate happens, although the prime minister may (or may not) resign, a "caretaker" government takes over until a new elections are held.

Here, it sounds like one of the four coalition parties has pulled out due to disagreeemnt over immigration, which the coalition parties had never agreed on.

No, but that's pretty common for them, they spent 2021 without a government because of this same guy: https://www.dw.com/en/netherlands-dutch-parties-to-form-government-9-months-after-vote/a-60109396

He did it in 2017 too.