What is a fast, universal way to launch a particular app (from within any other app)?
Besides the hardware functionality you mention in the last paragraph of your question, I think the only other solution is to use the NFC support in Shortcuts like this:
Open up the Shortcuts app and tap the Automatics tab in the bottom of the screen. Tap the &quot;Create Personal Automation&quot; button and then choose &quot;NFC&quot;. Tap the &quot;Scan&quot; button and hold the NFC tag on to the phone. Then tap &quot;Next&quot; and &quot;Add Action&quot; and choose the app functionality to launch.
Now you can use the tag to trigger the launch at any moment.
NOTE: You need an iPhone XS or later that supports background tag reading.