Operation coins are tied directly to your activity during limited-time events. If you want the full picture,
cs2 wiki has a great explanation of how each coin tier (bronze, silver, gold, diamond) is earned. Basically, you buy the pass, complete missions, and as you progress, your coin upgrades. The thing is, once the operation ends, you can’t upgrade the coin anymore — which makes the higher-tier ones a badge of commitment. Some older diamond coins are now collector’s items. I missed my chance to go past silver once because I didn’t know the time frame. If you want to be ready next time, that wiki will help you plan ahead.