The first tab when you choose the action for a button - is App. Here you can choose the App that you want to run when you press the remapped button. The tab contains all installed apps that can be opened.
But there is one more thing - activity chooser. You can run the app in the needed activity if the app allows it.
You need to hold on to the needed app, then the choose activity in the dialog. For example, we will choose DownloadActivity in MiXplorer:
!! warning "Reset & do nothing"
You can see here two special actions - "Reset" and "Do nothing". Use "Reset" when you want to delete the chosen action and "Do nothing" when you want to do nothing. It can be useful, for example, if you don't like that single press action runs immediately if double and long presses do not exists.