Help just bought a fit and it's rusted out!
 

Does anyone know much about the frames in these cars? I just bought an 07 Honda fit had the usual Honda rust I was going to go through and clean it detail it I noticed the rear floorboards were wet and they rusted out. They started digging around more I noticed the side rail frame that the plastic cover goes on is missing a chunk. I was wondering if anyone knows how much of the frame support is on the outside and how much of it is more towards the center of the car I'm worried the thing is going to split in half on me.

In a unibody car, the floor is the frame.
You can provide some pictures here for people to look at, but a better approach is to go to a mechanic for an inspection. In the future, get an inspection or check more carefully yourself before you hand over your money.

Yeah I checked under the car and it looked alright but they had sprayed the under body with some sealant to hide it all and the plastic panels had the rest hidden. Plus it was late at night so limited visibility. They took some serious step to hide it all. It was definitely one of those two good to be true deals only had 100,000 miles on it For $2500.

Not sure where you're located and what the inspection rules are, but it would be worth the money to have a mechanic check it out, both for your own peace of mind and also to avoid a failed inspection.

So good news I found another fit for $500 that's high mileage and in running order but it doesn't have a title so I'm gonna try to swap the motor over. In the meantime I'm just gonna swap the vin plates so I'm not worrying about my car doing the splits at 70 mph 😆. Now I just gotta find out how hard it is to do a manual conversion. I can't imagine it would be too terrible having a donor car to pull everything from.

Again, no idea where you're located but at least in the US it's a felony to mess around with the VIN (vehicle identification number).

If you're talking about swapping the vehicle license/number plates to a non-titled/registered vehicle that's also a legal no-no.

As for swapping engines, there are plenty of threads here about doing it, as well as plenty of Youtube videos showing various engine swaps on Fits. Transmission swaps are a bit more complex because there are differences (e.g., oil pan, I think the ECU) between A/T and M/T cars, but it is doable if you have the space and tools.

Swapping into a no title Honda Fit is an apocalyptically bad idea.