You do not need an app store for this. Open Jym in the browser, allow the camera, and the count happens on the phone.
Set the phone down
Put it on the floor in front of you, tilted up so the camera can see your head, hands and feet. Face the screen. The skeleton turns green when the lock is good. Reps start after that, not before.
Do the set
Lower until your chest is the low point, then press back up to a lockout. Jym counts that as one. If the line breaks — hips, depth, shoulders — you get a short note, not a lecture.
What stays on the phone
The video is not uploaded. A recording, if you turn it on with Pro, stays in the browser on that device. One counted set a day is free to start. Pro is there if you want more sets, the voice coach, or the replay.