TL;DR The original Bitcoin consensus was pure Proof of Work…

TL;DR
The original Bitcoin consensus was pure Proof of Work:
a transaction is final if it’s included in the chain with the most accumulated work.
So if we trade 1 BTC, no one can refute it unless they produce a chain with more PoW.
Modern BTC adds new consensus rules enforced by nodes, so validity no longer depends only on PoW but also on which rules people agree to run.
That’s why BTC is not Bitcoin under the original consensus mechanism.