The notification will be sent to the device which has the last new session (eg. last time the app was opening in a new window/tab). For apps installed on home screen the behavior varies. On Android, a new session is started whenever user opens the web app (or browser) after closing. On iOS web app, no new session is started when opening the app after closing.
This includes the native app. Both on iOS and Android a new sessions will be started when the user closes and re-opens the app.
What happens if I use multiple devices? Will I receive notifications everywhere?
Behaviour of push notifications when multiple devices are used by the same user.