The goal of money laundering is to take cash and turn it into valid-seeming bank deposits. So for example, they ran a motel, claimed to be taking in a lot of money and instead of having many paying customers, took money out of the bag and deposited it as receipts. You buy a 50-AC order from the mob-fronted business, but that business only has to purchase 5, send him real units (which require spending actual cash) and Marty sends them money for 50. Now that business has 45 AC’s worth of money in the bank, allegedly from purchases, so seeming legit. The mob boss gets a cut as profits etc.
When cash volume got bigger, the perfect fake money source was the casino. The casino takes in, say, $1M a week, they say they took in $5M, deposit $5M in cash, and now there is $5M of money that’s perfectly legit, sitting in the bank, to pay the shareholders (presumably Marty and the mob).
If you saw Breaking Bad the car wash was another source of cash deposits, and in one scene Walter’s wife is punching in deluxe car washes into the cash register as fas as she can - still not making much of a dent in the pallet of bills.
The problem is a careful audit will show - there’s no way you ha that many gamblers/motel guests/ car wash customers. The AC company would obviously not have bought 50 AC units from the manufacturer. Etc. The trick, then, is to not attract attention from the auhorities.
The rule of thumb with a lot of money laundering - allegedly, from stuff I’ve read - is that you can get about 50% out of the process, and the rest is “expense”; like those 5 real AC units, or the cost of operating a casino, paying staff in a motel, … and bribes etc.
They had a strip club too, cash business. You just inflate the number of dances or drinks you sold and deposit the illicit money.
IIRC the volume of money being too much for the businesses they were running was a plot point? Like you need the casino because your smaller business draws attention if it’s selling 5,000 bottles of champagne a night.
Marty initially had no cut. He owed that $8 million, which is why the first season they were always hard up for cash of their own. All of it was earmarked for the cartel as proof of concept. The later cut for the casino plan was 3% I believe and I think at one point they offered part of that to their allied political fixer (I forget how that panned out - I really liked the first few seasons, but kinda checked out thereafter).