I'm most familiar with the situation in Germany [1] where you either have to pay your fines, prove that you can't pay (in which case the fine is deferred or split into smaller installments) or get detained in prison for up to six weeks (or three months for multiple fines).

Of course most people do end up complying in one way or another (which means that the fine is eventually paid, even if very poor people might need many small installments).

I'm pretty sure that most other countries will also eventually threaten to imprison someone if they make no effort to pay a fine, otherwise many people would simply ignore them and never pay.

[1] https://www.gesetze-im-internet.de/englisch_owig/englisch_ow...