In short you have (making these terms up):
1. Growers who produce and distribute the weed,
2. Runners who pick it up and bring it to the coffeeshop,
3. Coffeeshops
It's only legal for the coffeeshops to sell.
This sometimes devolves to a game like tag. The runners need to go as fast to the coffeeshop as possible in the most stealthy way. If the police catch them, then they are in trouble. However, if they bring it into the coffeeshop, they are safe.
Disclaimer: I am very biased here, I think this whole system is idiotic. The Netherlands should legalize weed. They have two economic incentives (1) weed tax and (2) sustained tourism.
[1] https://www.rijksoverheid.nl/onderwerpen/drugs/gedoogbeleid-... - official source
[2] https://www.jellinek.nl/informatie-over-alcohol-drugs/cannab... - some talkshow source but explains it quite well