How can I fill this gap between two areas of flooring
Upper Picture - there is a saddle, but it is not doing what it is supposed to do - transition fully from one room/floor to another.  You could remove that saddle and get a wider one that covers the full gap. An issue may be the relative height between the two areas.  If they are uneven, you might need to put a filler under the saddle that levels them.  This should be a molding made of the same wood as the saddle and stick out slightly beyond the saddle on the lower side (maybe 1/2" to 1").  it needs to be tapered to avoid a sharp transition.

Lower Picture - There does not appear to be any saddle creating a close seal between the bottomn of the door and the floor.  This is an area that needs attention to avoid insulation problems, moisture seepage and even insect incursion. There shoudl be a saddle that reaches the outside floor level. Again, uneven levels require some build up generally under the main saddle. The gap is less of an issue than bridging the gap.