The operator would publish a Bitcoin address and the spreadsheet. Participants would send whatever amount of Bitcoin they wanted to this address. The operator would add deposits to the spreadsheet in the order received.
Starting from the top of the list, as soon as the total deposited was enough to double the first deposit, the operator would send double the amount to that participant.
Then when total deposits grew enough to double the second deposit, it was paid out. This continued until there weren't enough new deposits to double the next in line.
Could be illegal, but it's definitely not a scam because everyone could see from the beginning that the pyramid must eventually collapse. Many of these games were actually called "Ponzi" in the name of the spreadsheet.
Some of these games ran a surprisingly long time, months in some cases.
And in the games I played, the anonymous operator never pulled an exit scam. They profited naturally when deposits eventually dried up and the remainder was their reward.