Nook Shopping, which is normally accessible via the ABD at the Town Hall in Animal Crossing: New Horizons, could apparently be accessed via your in-game Nookphone.
In order to unlock the Nook Shopping app on your Nookphone, you will have to make at least 100 purchases via Nook Shopping.
As there’s a limit of 5 orders a day, you’ll need to order 5 items for 20 days in order to unlock the app.
This could be tracked via the Nook Miles challenges section.