An iframe inside an iframe inside an iframe — the target button is three context switches deep.
You'll need to switch into each iframe's context in sequence — most frameworks won't search inside nested frames automatically.
Playwright's frameLocator() can chain straight through all three levels; Selenium needs switchTo().frame() three times, with switchTo().defaultContent() to reset if you back out.
"Deeply nested button" is three <iframe> levels down. Switch into each frame in turn
(level 2 → level 3 → level 4) before you can locate and click it.