they'll have numerous rounds of voting, no one including McCarthy will have the votes, then the Dems will offer to support McCarthy (or someone else they think they can work with) in exchange for a few demands (aid for Ukraine, no major cuts to social programs, etc.). new speaker takes the gavel, they pass a boring steady-state budget, and the House proceed to do exactly nothing* until the next congress is sworn in, in january 2025. the end.
* "nothing" here includes the Biden impeachment inquiry, which will lead to nothing (other than possibly embarrassment for the GOP...)