AsteroidOS watches can be synchronized with a variety of devices given that they install a "synchronization client". This page lists all the known synchronization clients by platform.
AsteroidOSSync Android application
An official synchronization application for Android phones named AsteroidOSSync is available to download on F-Droid.
This app currently offers the best experience possible with AsteroidOS.
Gadgetbridge Android application
The AsteroidOS Gadgetbridge integration is considered to be in initial stage. Nearly all features provided by AsteroidOSSync are supported. Only the remote volume control and screenshot functions are missing.
There is currently no application available for iOS phones to synchronize with AsteroidOS watches. As far as we know, there hasn't been any development done on such an app yet.
However, if the watch is paired with an iOS device, AsteroidOS is capable of displaying notifications from the phone. Pairing can only done using 3rd party BLE scanner/communication apps like nRF Connect as BLE-only devices do not show up in iOS Bluetooth settings. Specific functionality like syncing time or weather forecast are however not supported!
Ubuntu Touch/UBPorts application
A synchronization app for Ubuntu Touch named Telescope has been started by the community. You can download the app in the OpenStore.
This app tries to re-use as much code from the SailfishOS client Starship as possible. Some of their codes is shared in a daemon named asteroidsyncservice which uses libasteroid. The app is still under development. First functions like notifications, time syncronisation and battery status reading are already implemented.
A community member created a desktop Linux synchronization app named AsteroidOSLinux. It is written in Python and offers some basic scripting capabilities for AsteroidOS synchronization. The code is written in a way that makes it very easily extendable for your own usages.
A demo of the libasteroid usage named asteroid-ctrl is also available as a C++-written CLI application. This app is cross-platform and doesn't do much on its own but can also be extended for your own usages.
A library named libasteroid encapsulates the Bluetooth Low Energy profiles of AsteroidOS in a simple API. It is written in C++ using QtBluetooth and is then cross-platform. Its code can be used as a reference implementation for other clients development or as a 3rd party library.