A platform advertises "cloud mining" or "staking" services, offering to run mining hardware or validator infrastructure on your behalf in exchange for an upfront payment, with promised fixed daily or weekly returns.
In reality, there's often no actual hardware or validator infrastructure behind the platform at all — the dashboard showing your "earnings" is simply a number the operators control, not a reflection of any real mining or staking activity.
Early payouts (if any occur) are usually funded the same way a Ponzi scheme works, from other users' deposits, until the operators stop paying out entirely and disappear with the remaining funds.