Basically, at the point of the merge, nodes stop paying attention to what the PoW calculations are saying about the head of the execution chain, and start to pay attention to what the beacon chain (PoS chain) is saying about the head of the execution chain.

Because the two chains become inextricably linked together at that point, with the execution chain becoming entirely dependent on the beacon chain, we call that point the "merge".

So from that point on, new blocks on the execution chain - full of general Ethereum transactions - will be considered "confirmed" not because they include a difficult PoW calculation, but because the beacon chain has given that block its blessing (because stakers voted on it).