Tell me how in Ionic (I use Angular and Capacitor) to create function that, after building project on Android, will work even when application is closed (including after restarting the mobile phone)? It is required to make periodic POST requests to server and receive data (with open and closed application). If data on server is refreshed, I would like to display LocalNotifications from this background service.
Your question is composed of several questions.
There is an Ionic plugin that can do this. Autostart
You can use the Backgroud mode Ionic plugin.
OneSignal can display notifications, sent from your backend, on your smartphone. There is also a Ionic plugin for OneSignal. The plugin will take care of being active even if your app is not and will also always be up, even after restart.