I'd also bet that the vast, vast majority of voters are already going to vote for their chosen candidate, independently of whether they see $1 billion worth of ads or $5. If anything, "free" advertising like going on a podcast or working at McDonalds as a stunt seems to have more influence.
This is true of elections in two party systems. Most people have parties they align with and don't switch often. But there are persuadables.
The billions spent on political ads was spent for a reason. Similar to why billions are spent on marketing in general. There is the old adage that sure half of the marketing budget is wasted, but it is never clear which half ahead of time.
https://www.b2bmarketing.net/half-the-money-i-spend-on-adver...
It's about convincing your people they need to show up or the other side will make your grandkids shit in litter boxes, or whatever lies it takes.