Linux Applications

Linux applications that work well with iPhone.

iDescriptor	Cross-platform, open-source and free idevice management tool written in C++
Cider	Cider is a new cross-platform Apple Music experience built on Vue.js.
KDEConnect	iPhone file sharing and lmanagement built with Qt and fits well in KDE. Requires KDEConnect from AppStore.
GSConnect	iPhone file sharing and management built with GTK as Gnome extension. Requires KDEConnect from AppStore.
digiKam	Photo management with iPhone sync.
KeepassXC	Password vault. Database can be synced via file share services to many AppStore compatible apps.

iPhone/iPad Applications

Applications on Apple devices that make data usage convenient.

Flacbox	Music player that can access Samba. You can also use the built in web service or directly connect to Linux and drop files. Plays flac and ogg files.
Yomu	Ebook reader and manager. Can connect to Calibre OPDS or be connected directly to Linux for file drops.
VLC	Video and music player.
AuthPass	Keepass compatible password vault. See KeepassXC

Applications On Both Platforms

Applications available on Linux and iPhone that sync and share between devices automatically.

ProtonMail	Secure email. Commercial.
Session	Secure/Encrypted chat.
Joplin	Notes and Tasks. Can be encrypted and synced.
Simplenote	Notes everywhere.
Enpass	Password manager. Commercial.

I know there are many more, but I have used these and have found them productive.