There's no JustOnce app to install — open app.justonce.ai in Safari or Chrome and sign in. To get back to it quickly, add it to your home screen (Share → Add to Home Screen on iPhone; the menu → Add to Home screen on Android).
On a phone the sidebar becomes a bottom bar with the main areas and Search, plus a More button that opens the rest: the remaining pages, Invite a friend, Help & support and Log out.
Connecting an assistant is easiest on a computer. Once it's connected, a connector added on claude.ai also works in the Claude mobile app.