What happens to any $$ left over after the elections?? does the candidate get to use it for any thing they choose, or must it stay in a reelection account???

They use it for winding things up and can donate it to charity. There are other possibilites as well. It is not to be spent for a top of the line AR.

The congress critters have to make sizable “donations” to the party elites to obtain their committee appointments. My guess would be much of that money comes from leftover campaign donations.

Officially, they keep it for the next election, donate to other candidates election, etc.
I suspect in reality there are relatives on the campaign payroll and some gets siphoned off for other uses.
Somehow they seem to go from public servants to multimillionaire status without holding real jobs outside of their official salary.
Much of it comes via the Hunter Biden model.

They are pretty much discretionary slush funds. They just need good accountants and lawyers.

The money gets funneled to other candidates. It’s how Pelosi built her base of power. Years of raking in campaign cash in a district where she didn’t have to spend a penny. She sent her money to other members of the house and when it was time to grab power she called in their votes.