You're mixing a lot of concepts into a few words so.. if I may, let's standardize the terminology.
First, Hubitat hubs split on the concept of "admin/user": adding devices, adjusting rules, adding devices to specific dashboards... all of that and more are "Admin" functions. User functions are largely Dashboard button presses and their miriad variations on the theme. Anything that changes the configuration OF your hub is admin, anything that changes the condition of the devices attached TO the hub is user.
User functions are accessible remotely (not from your own home LAN/WiFi) via Dashboards and dashboard like operations. There are zero Admin functions available remotely.
To get Remote Admin Access, you need a VPN you build yourself from recipes on the internet, OR you subscribe to Hubitat's Remote Admin package.
The App is a user tool. It is one of the "miriad" and "dashboard like" items I reference, as are Apple Homekit, Amazon Alexa and Google Home. It allows you to turn lights on or off or set dimmers or check sensors, ("Did I leave a door open?") and so on. What you CANNOT do via the app is any of the admin functions.
Didn't add a a device to a dashboard? You can't do that via the App. You can use Hubitat's Remote Admin package or, for free, your own VPN, to modify everything ON your hub.
I don't know if the answer to your questions are in these words, but perhaps a concept or two will help formulate the question with more clarity?