TL;DR: Editing this file works:
C:\Program Files\Blender Foundation\Blender 2.83\2.83\scripts\addons\io_scene_obj\__init__.py
Long version: How to edit the file:
This file is a text description of the UI elements a user sees when selecting file > import > wavefront (.obj).
If you look through the file, you will see
class ImportOBJ and within that class, several blocks of text called "BoolProperty" or "StringProperty" or "EnumProperty".
Each of these blocks of text corresponds to an element you see in the "Blender File View" window operator presets panel:
In my case, I was looking to alter the default value for "split/keep vert order" (because changing vert order on import is very unwise, as it's destructive), which was "ON" (split) in my distro (2.83.10). I changed it to "OFF" (Keep Vert Order):
items=(('ON', "Split", "Split geometry, omits unused verts"),
('OFF', "Keep Vert Order", "Keep vertex order from file"),
You can use this technique to change the way the UI appears in many ways. This is just one small example, specific to this question.