The Journal app on iPhone, which is finally available with iOS 17.2, is a great way to build a journaling habit. With a digital journal, you also get the additional benefit of receiving notifications that'll nudge you to document your thoughts or reflect on ideas.
You can allow notifications from the app when setting it up initially or later from the Settings. But the notifications are delivered at a schedule that the Journal sets up on its own. But when it comes to something as personal as journaling, the schedule cannot be "one-size fits all" as the app initially