I’m syncing almost all data via Nextcloud. That includes actual files as well as contacts and calendars. The files are obviously on my iPhone, but not in iCloud. In fact, iOS makes CalDAV and CardDAV as easy as they could be. It’s natively supported, whereas Android requires an extra, paid for app (worth the money though).

Other synchronisation like Joplin and Zotero happens via WebDAV. My iCloud is basically empty yet I have every file I could ever need on both iOS as well as iPadOS. Some apps I don’t care for sync via iCloud, that’s all so far. I’m not bought into the whole ecosystem (i.e. apps) too much though. If all you use are apps that only support iCloud, that’s a problem indeed.