Because the “In-App-Browser” on iPhone and Android are not full featured browsers and because our App Content pages are created and stored on WordPress we have to live with some limitation. Namely, we don’t have access to the <head>…</head> section of the page.
I have had good success as long as I stick to vanilla JS and jQuery 3.x. I will update this as I verify other working functionality (libraries & frameworks).
External scripts can be called/included by both the AJAX and Concatenation methods as seen in the page linked below.