It's usually just not declaring profits in my experience, it's brutally addictive to slap 30% ish profit on top of everything.

My boss at a pizza place had 2 registers he would use for various reasons but it all boiled down to one being reported to the IRS and the other not.

I know a liqour store that has a small bar inside thats cash only, I got friendly enough with a manger that basically admitted the locals that hung out there were mostly blue collar and came in after work where they got payed cash. Meanwhile the retail liqour customers largely were card users. So the owner made the bar cash so he could report less. Makes sense why its the cheapest bar around by a large margin.

Not declaring all your profit is actually the opposite of laundering money, so it's going to be one or the other, never both