Filen - Server in Germany, run by Filen. Has clients for Windows, macOS, Linux (both x86_64 and arm64), iOS, and Android; but not BSD or ChromeOS. You get 10 GB for free.
Nextcloud - Server wherever you want, as you host it yourself. Has clients for most platforms, including Android, and also supports WebDAV. Everything is on your terms. Also comes with things like a calendar, a notes app, and ActivityPub (I think). You can run NextCloud on a Raspberry Pi under your desk for cheap.
Syncthing - No server and no account, as it is strictly peer-to-peer. Has clients for Windows, macOS, Linux, BSD, illumos, Solaris, and Android; and there's an unofficial client for iOS (Möbius Sync). Devices must be on the same network to sync (although there's probably a way of getting it working globally). Totally free to use.
I use Filen because I only have one Raspberry Pi (which is in use), so Nextcloud isn't a viable option; and I had trouble getting Syncthing to work.