Google "wood floor transition strips". There are many, many types available. I have this type of transition from the higher tile floor in my laundry room to the lower wood floor in my hallway

There should be a matching stair nosing available to transition into staircases. You will need to cut the flooring back to fit it.

And why not customize a piece of flooring with a simple bevel to transition the two different floor height? It would be a tight fit transition.

Thank you. how much of a bevel would you revommend? just the edge or a longer slope? the new floor is down at that door. there is about a 3/4 to 1 inch gap between old and new. have another door where we haven’t put the floor down yet so may be easier to deal with.

For a 3/8” difference, I would do a simple 1/4x1/4” bevel at the edge. It’s nice and clean. it also, makes it easier to refinish in the future since, it’s a flat leveled surface.

In follow up, first thanks to @G & S Floor Service for the suggestion. really happy with how it turned out at the doorways. Sanded to round out the bevel edge and applied some finish. i am looking for suggestions for best way to keep it in place. Finishing nails or some kind of glue? Also, at one doorway the old floor is uneven and there is a gap at the edge (left side in the photo). is there some kind of caulking ir something else we could fill this with that would make it less noticeable and easier to kerp clean?