Despite what a new vehicle owner may imagine, some vehicles have poor sealant work - from the factory.
One of my cars had a pond underneath the right rear seat area. The welding job for the right rear wheel well, was mis-aligned and water easily poured thru the gaps and into the “tub” of the floor beneath that seat area.
When I pulled up the seat, I thought: “It might be damp?” It had 1 inch of water in it.
I resolved to yank out all seating and the carpeting, and then I “attacked the car.” EVERYWHERE. I applied sealant.
Next, I used high-quality marine engine room insulation and sound-deadener, plus spray-type-sound-deadener - EVERYWHERE I could place it.
Re-installed carpet and seats.
For 25 years I drove that car, and the occasional guest would marvel at how quiet it was.
Pick some portion that you are able to tackle, and win just that bit - one day, one week, one month at-a-time.