Conjecture: I suspect that Apple is underwriting or paying for the new fabs years in advance, since Apple has wads of cash overseas that it needs to invest (due to US tax laws). Apple can place orders years in advance for all the output of entire fabs, and by paying for fabs it would be able to control costs of exclusive orders for the production. Apple doesn’t want to bid prices up against competitors, so long term contracts make sense. TSMC would perhaps not be able to finance such aggressive moves to new nodes without Apple, and contracts can remove most financial risk for TSMC.

The above is just capitalism at work - the richest company in the world investing in the world’s most expensive toys. Apple has ~5x the market cap of NVidia and Apple can predict their usage of silicon, whereas NVidia’s market is more volatile.

Whether the above is monopoly power is arguable.

I also would strongly suspect that TSMC has political requirements embedded in the contracts. For example Taiwan needs phones that are secure against China (security against state level attacks), and Apple can meet needs that other fab users cannot. There is no doubt in my mind that TSMC are using Apple as part of their silicon shield: https://duckduckgo.com/?q=Taiwan+Silicon-Shield