90% of my floor is in very good condition but it seems like the pathway’s poly has come up and there is some dark areas.Can I screen the good part of the floor and sand the bad part?Or do I have to take the whole floor down to bare wood?Thanks

If it is your floor you could make it look better and last longer by doing what you suggest. If it is someone else's floor they would like it all to be sanded at the same time.
Sand to bare wood where it is dirty or has been affected from the atmosphere by not being sealed as the finish wears off. Two coats of finish to bare wood, then clean and apply the third coat over the whole floor, after sanding with 150 everywhere.

The color will eventually become pretty close.
Before the floor wears too much a coat of finish needs to be added where the finish has worn. Water based finish sticks to oil based poly and resists wear, but doesn't give the rich color. Area rugs will protect a floor that is starting to wear through until another coat or two of finish can be applied.

Would I be able to use Oil Base over oil base instead of water? Should I use 150 screen or paper?I was thinking about getting Oil base Varthane.I have some rooms that only need a re-coat and some with wear mentioned before.I guess I’ll sand the whole floor in the rooms with wear and light sand and re-coat in rooms with good floors.On re-coats ,what grit paper should I use, and how many coats on top of original poly? Thank You

Oil base will give the color when sanding to bare wood. After the second coat, hand sand the whole room and use water bas on the final coat for the bast wear. Use a 2 part finish, or oil based. which will add ambling to the color.
A 150 grit screen.