I think I’m looking at original floorboards, rotting underlayment on top of those, a delightful assortment of other boards on top of that, then Pergo flooring.
I’m unsure of the best way to proceed. My gut says to pull up the top layer of boards to remove that underlayment, but then I worry that that turns into ripping up the whole floor.
(Left Side cabinet)
(Right side wall)
I’m not an expert, but I think this is fairly typical. Unless any of that wood is actively rotting (which would indicate a larger moisture issue that needs to be addressed), using old ugly boards to stabilize and level cabinets and countertops would seem to me like the appropriate approach during installation.
It looks like this was a remodel? If so, they definitely wouldn’t have bothered to make anything look pretty if it’s going to be hidden behind cabinets.
In other words - it’s probably not as bad as it looks. If it ain’t broke, don’t fix it.