What could someone with more than 50% of the mining power attempt?

Detailed explanation

A 51-percent attack is a simplified term for an actor or coordinated group with dominant hashrate. Mining devices need not be in one place or powered by a single power plant. ASICs distributed worldwide can work together on the same alternative block history. What matters is their share of active total hashrate, not geographic concentration.

With enough hashrate, an attacker can create a private competing chain. If its cumulative work overtakes the existing valid chain, recent confirmations can disappear and the attacker's own payments can be reorganized. Transactions can also be delayed or censored. Without private keys, however, the attacker cannot spend other people's UTXOs or force full nodes to accept an invalid money supply or false signatures.

Whether an attack pays depends on hardware control, electricity and operating costs, duration, possible proceeds, price effects and objectives. A financially motivated attacker may want to avoid losses; a saboteur or state actor may be willing to lose money. Technical possibility therefore does not automatically mean economic sense.