I'm not a professional, but I was taught by one and have helped do the work on every home we've repaired/redid for 52 years. @DeVonSwi is right. Plywood instead of particleboard is definitely the way to go on flooring. I've seen particleboard fall apart far, far more often than plywood. Shimming is also the way pros used to level things. Cardboard shims disintegrate faster than wood.

That raccoon is a-round out there

Maybe they intended it to be a home for their tortoise during the nice months, but I'm going with fire pit, too. I'd use it and see how it works, but only after checking my insurance to be sure it's covered if a spark sets something else on fire. In the southwest we use chimineas a lot so I think this fire pit is a super cool idea if floating embers aren't a problem where you live.