Well they simply have gaps in regulation around elections. They tried to reform campaign finances with HR 1 For The People Act but it was never called for vote in the senate due to Senate Speaker Mitch “The Legislative Reaper” McConnell who was notorious for refusing to call things to vote.
One of the parties simply does not want to fix democracy.
The money gets spent on billboards, flags, yard signs, pamphlets, door to door outreach, renting out venues to campaign, etc.