I recently rewatched Doctor Strange and I noticed an interesting detail. The Ancient One explains that the Zealots have more power in the Mirror Dimension because of their connection to the Dark Dimension. This was the explanation as to why the Zealots (and the Ancient One) were able to morph the Mirror Dimension to their will, but Strange and Mordo couldn’t.

Additionally, the Russo brothers made sure to uphold these rules for the Mirror Dimension in Infinity War and Endgame. In those movies, we never see Strange or Wong morph reality within the Mirror Dimension (because that would require a connection to the Dark Dimension).

However, in No Way Home, we see for the very first time Strange manipulate the Mirror Dimension while he is batting Peter. This can only mean one thing: Strange is able to morph reality within the Mirror Dimension because he now has a connection to the Dark Dimension. Just like the Ancient One, he must be drawing some sort of power from that realm. This would be the only way he could manipulate the Mirror Dimension.