Session replay is for Web only. Native mobile (Android, iOS) session replay is not currently supported but is on our roadmap.
Session replay is an add-on feature for paying customers only. You may try the feature for free for 14 days or 10,000 sessions (whichever comes first), after which you must contact Sales to continue use.
Yes! When you implement custom client-side tracking (ex. heap.track()) we add session replay metadata so that you can watch replays for those events.