Every time you move more than $10,000 into or out of a bank account in a 24 hour period (presuming Canada or the US) your bank will report it to your country's financial intel agency.
They notice when a cafe is pulling in far more money than it ought to. Then they hand what they have over to the IRS/CRA, FBI/RCMP, and let them investigate further.
If you're moving any significant sum of money, you're on their radar. And they aren't stupid.
The real money in is stuff like real estate, which is pretty shady even when legit and much easier to buy and sell
It's like this: I buy a house for $500,000. What a great deal. A year later, I sell it for $600,000. Hey cool, I made $100,000 profit. Of course, quietly I've handed the original seller a briefcase with $110,000 cash in it. The extra $10k was to keep his mouth shut, but in return I've converted $110,000 of dirty money into $100,000 of clean money.
But maybe it's not just one house- maybe it's 10 houses. Maybe it's a huge mansion worth $5m (which I negotiated a great deal on at $4m). I'm not cleaning the full value of the asset, I'm cleaning some portion of it's value.
What those other parties do with the dirty money is up to them, but it's easier to deal with smaller amounts of it, spread out, than it is to deal with one big pile of dirty money. Maybe they work construction as a general contractor and buy their supplies for jobs using cash, but get paid in clean money. Many hands make short work when it comes to cleaning money.
All that said, every large purchase - homes, cars, boats, etc - leaves a trace. Financial Intel agencies get a report each time you move $10k or more. And if you move too much, too frequently, they start investigating- how come this guy bought 5 houses in 3 years, each one below market value? Sooner or later, someone under questioning by the police gives you up and then it all unravels.