That is strange. Could it be termites? A poor seal under the door?

You should shine a flashlight up into the ceiling and also map out exactly where the rectangle is above. The way to do that is to have someone tap the ceiling in the basement, while the other person puts masking tape on the floor above. Those should give you answers we can't. I could speculate, but it would be a waste of time.

Perhaps, with high quality pictures up into the ceiling and the outline taped above, we might be able to help, but that should probably tell you all you need to know without help.

Dirt getting in the crack between porch and wall. Door shutting vibrates the entirety and knocks the dirt down in the crack and it has an opening through with how it was framed (can't see how it was done from here).

The dusty or gritty debris appearing in your basement directly under the front door is likely a combination of external dirt, road dust, and deteriorated materials from the entryway above, entering through gaps in the structure. This is a common issue caused by a lack of a proper air or water seal at the front door's threshold.

AI response, but exactly what we have going on, except it is at our garage door, over basement. Thanks for posting. It may solve our problem, as well.