Imagine the following scenario: On the 9th of January 2024 in the morning we wake up to news that President of the USA Joe Biden suffered a stroke and died in White House. The administration confirms that the death was entirely natural and no third party contributed to it. Even before the news reach all TV stations another one breaks: Former president of the USA Donald Trump has suffered a cardiac arrest and died in his residence in Mar a Lago. Early reports claim that the site was secure and no malicious actor could reasonably reach Mr. Trump so the death is most likely natural.
What happens then?
25th amendment describes this process. Harris selects a new VP. Both houses of congress confirm by majority vote (good fucking luck).