The app atop the list is the one you used previously, or the first installed app. To use that app all the time and never be bothered again with the prompt, tap ALWAYS.
To use the top app and see the prompt again, tap JUST ONCE. Otherwise, you can choose a different app from the list, which is opened "just once" for the current task. Next time, you see the prompt again.
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My advice is to choose JUST ONCE until you get sick of seeing the Open With card. At that point, choose ALWAYS and set the open-by-default app. The fear, of course, is that you’ll make a mistake.
- The open-by-default app is different from what Android refers to as "default apps."