Right of the photo, you can follow the stairs with the base molding. Cut a recess in the tread so that mold can slip in. Then fill, sand and paint. Cover the middle with some kind of tread protector or carpet.
If you need finished stairs, I would recommend a new stairs.
Left of the photo, are those cuts in the tread and risers? Fillers? Those may not be safe.
Adding 1/4 or 3/8 laminated floor can work, but you have to know how they change the riser heights, esp for the bottom landing. You cut the nose flush to the riser, put new nose/tread and new riser cover.
Composite tiles work as well with steel nose cover.