Looks like 1x4 T&G Fir. It also looks like old growth, tight grain which is cool. You'd have a hard time buying that stuff these days. The old stuff was generally long boards too so fewer butt joints. It's rough sawn under the cabinet but could have been sanded and finished in the main room. If I were you, after removing the plywood, I would pop up several tiles (carefully not digging into the Fir ) and test sand at least a 3x3 area. Wet it down and you'll see what it will look like finished. You'll also find out how to remove the tile adhesive with the test spot. If it seems viable to do the whole room rent a drum sander and edger and sand it with 80 grit. Vacuum thoroughly including the all the seams and voids.Then buy a gallon of filler and using a 6" flexible sheet rock knife (putty knife) fill all the joints. Don't try to level the filler, leave it standing proud. After sitting over night sand with 120 and apply sanding sealer. Let sit over night again and apply 2 coats of polyurethane finish. I would suggest water based in satin. Anything shiny will really expose sanding errors and everything else. Pictured below is what I use but any sandable wood filler for Pine or Fir will work. Good luck. It could turn out to be a really nice floor
