Learn/The device itself
What a phone can do that a website cannot, and what it asks of you.
The reason to build an app at all. A browser tab cannot put a widget on a home screen, wake a phone with a notification, or keep working in a tunnel. Everything here is a capability you only get by being installed.
Start hereHow do push notifications actually work?What a notification really is, and why both stores now make it easy to refuse.8 guides, grouped by what you are trying to find out.
How do I reach someone who is not in the app?
Notifications are the capability people install an app for and the one they uninstall it over. Both stores now make the permission easy to refuse, so the first ask matters.
How do I get onto the home screen and the lock screen?
Widgets are the cheapest presence an app can buy: visible without being opened. They are also built differently on each platform, so there is no single answer.
How does the app open in the right place?
A link that opens the app on the correct screen, from an email, a message or another app. Easy to add, easy to get subtly wrong.
What about screens that are not a phone?
Cars and dashboards. Both platforms gate these behind an application rather than a setting, so the lead time is the real constraint.