When a floor is a different level the pros use a transition strip. These are made from the flooring or any block of the same species of wood. The general rule is that it should be a gently tapered flush fitting piece that eases you from level to level.
There should be no sharp angles or butt edges that can be caught by foot traffic. Our goal with a transition in wood flooring is to avoid causing people to trip. It should be subtle and finished with the same color as the floor.
The upper part of this floor is Beech, and the lower right hand side is pine. We used a custom tint to make them look similar.
When you cannot match the finish… you may wish to add contrast. Give it a darker stain on the transition if that will help make it look better.
You can use countersunk screws and tapered plugs glued in to cover the screws… or PL Premium adhesive and unobtrusive finish nails to hold it in place until the glue sets.
Tapering the cuts on the butt end of the threshold will help you get the joints tight. Typically you mount the threshold temporarily, then butt the end grain cuts up to the threshold.
Shown below are samples of threshold where it meets ceramic tile… those need to overlap the tile to protect from damage from foot traffic.
This type of threshold can be milled on a table saw and sanded smooth with a belt sander/palm sander. The harder the wood, the more aggressive the tools you will need to finish it.