there is a preferred app, not sure if you must use it but from my experience it syncs with the steadicam

Yes, you must use their app and the video is stored in the app, not in the photo folder!

SteadiCamSDV-VOLT Electronic Handheld Stabiliser for iPhone 7/7 Plus and GoPro HERO5 Camera - Black

Q: Are there camera controls / functions built into the steady cam volt: shutter button, focus, zoom etc. ?

Tiffen promised an "ImageMaker App" to turn off the camera stabilization and a few other features. Like the Volt, Tiffen hasn't delivered. I ordered the Tiffen Steadicam Volt on Kickstarter. Tiffen had a few delays as expected in any new product development. Breaking their own promises, they sent inventory to retail before the 6,000+ backers who collectively spent $1.1 million. Tiffen charged Kickstarter backers extra for a "premium" case, but threw it in free to pre-orders on their website after Kickstarter closed. I have bought a lot of Tiffen gear over the last few decades. I lost all trust in this company.

Q: Attachable to a Selfie stick to use as a jib? I hope the handle can be taken off and then screwed to a 1/4 or 3/8 screw thread.

In my experience, the Volt is not stable enough to be put at a distance from one's fist and fingers, which are needed to help guide the camera and keep it from wandering. I also think that an extension stick would multiply any bobbing effect (from which the Volt is not immune) as you walk or run.