Does anyone know how online stores eCommerce in store pickup knows my location?

Stores with online Instore pickup put the distance/mileage they are willing to allow someone to place an order so someone out of state doesnt place the order and can't pick it up. If I chose a place 100 miles away and chose in store pickup it would say I'm not eligible but how do they know when a buyers address isn't entered?

I thought it would be by IP address but IP location is not reliable. If you check your computer IP location compared to your mobile IP location when they are sitting side by side, they may claim 100+ miles apart.

Stores with this feature can say eligible for pickup within 20 miles in their settings BUT what finds buyers location?

If stores go buy zip code, how do they find what zip code you are in?

If stores went by zip code, I would not think they could get exact on mileage such as 23 miles.

Stores don't want buyers address for location verification because they can make up fake address close by and truely be 1000 miles away. Stores need the truth and they do get it (somehow).

Any idea how this info is gotten?

Thanks so much for this info in advance. It will help with my site.