First you need to choose a central device where you're going to send all of the important stuff from other devices like smartphones, laptops, etc. Then you need to setup Syncthing, which works on linux, macos, windows and others. For android there's Syncthing-fork[^2] but for iOS idk.
Setup the folders you want to backup on each device, for android, the folders I recommend to backup are DCIM, documents, downloads. For the most part, everything you care about will be there. But I setup a few others like Android/media/WhatsApp/Media to save all photos shared on chats.
Then on this central device that's receiving everything from others, that's where you do the "real" backups. I my case, I'm doing backups to a external HDD, and also to a cloud provider with restic[^3].
I highly recommend restic, genuinely great software for backups. It is incremental (like BTRFS snapshots), has backends for a bunch of providers, including any S3 compatible storage and if combined with rclone, you have access to virtually any provider. It is encrypted, and because of how it was built, can you still search/navigate your remote snapshots without having to download the entire snapshot (borg[^4] also does this), the most important aspect of this is that you can restore individual folders/files. And this crucial because most providers for cloud storage will charge you more depending on how much bandwidth you have used. I have already needed to restore files and folders from my remote backups in multiple occasions and it works beautifully.
[^1]: https://github.com/syncthing/syncthing [^2]: https://github.com/Catfriend1/syncthing-android [^3]: https://github.com/restic/restic [^4]: https://github.com/borgbackup/borg