Linux

As others suggested the backend is probably already installed on most computers but not setup, namely :

• ssh to manage passworldless across multiple computers (you need that for data to be safe)
• scp/rsync/rdiff-backup to actually copy the data thanks to ssh keys

One could imagine a dedicated user per machine that is for read-only of data (maybe after some encryption, limited to very specific directoriess) and another for storing only of data (with no access except to write on disk and with a maximum quota).

What this highlight though is that the centralized managed cloud model is challenging to replicate as purely p2p at home, namely backing up your phone to your desktop might be find but the other way around, probably not. Maybe even more challenging, what do you actually backup? I would argue your home directory but... clearly not your e.g. Steam games (humongous) or other backups or video files downloaded from the Web. So... probably a select set of directories in home then, but which ones? ~/Documents only? This specific part implies some decision from the end user.

Anyway I believe all the tools are there, but I think what most people lack is to view the result and for that maybe some equivalent of https://gitlab.com/ikus-soft/rdiffweb/ which shows when was the last backup done, how big it was, etc basically some form of visual to feel safe.

Finally to skip the CLI key management part the closest I know, for end users, is KDE Connect https://kdeconnect.kde.org/ which I discovered after building my own https://git.benetou.fr/utopiah/offline-octopus kind of equivalent, namely a way to use devices on LAN. Backup is not a default feature though but could be.

[…] just as convenient and beginner-friendly as what Apple provides?

There's a reason why Apple is able to charge so much money for that — and that reason is that the answer to your question is no.

iCloud shit is only 'convenient and beginner friendly' because you're paying someone else to do all the work and maintenance for you.

Nextcloud & Syncthing

Every system has its own processes. If you want Apple software and services use Apple. If you want Linux use Linux. Do not expect either to be like the other especilly at such a micro level.

As far as Linux and beginner friendly, buy a device with Linux preinstalled just like you do with Apple. As far as user setting and apps. Get a notebook and write them down, and avoid deep customizations. As far as backup get 3 USB drives and backup your home directory with rsync or one of the other solutions. As far as restore, have install media and just reinstall from scratch then layer in your configs and apps and then restore your home directory files. For file sync and app sync functions, Nextcloud is helpful and you can pay for a commercial host, set it up yourself, or use a product like Synology. You frankly could use Dropbox, Proton Drive, or one of the others also. But think carefully what is actually needed. Cloud stuff is heavily promoted by the big providers presumably for lockin reasons and to mine your data but it is not really needed for most things. Get to know your distros builtin emergency startup tools and have a live distro like the live install media available and know how to use them.

Linux is about options but for simple beginner like processes it is best to stick to the basics.

Oh, and there is also bup, which might be what you are looking for:

https://bup.github.io/

• it stores files in version-controlled copies which can be synced. Perhaps good for backing up photos and such, up to a few GB.

Two more interesting solutions:

1. Nix OS and Guix SD let you define a system entirely from single configuration file, so it is easy to re-create when needed.
2. The Btrfs and ZFS file systems allow to take snapshots in an instant which can very efficiently store earlier versions of files. I used that when working with yocto/bitbake, which compiles an entire embedded system from source - it can handle much larger data volumes than git or bup, and is the right thing when handling versions of binary data.

And one more, the rsync tool allows to store hard-linked copies of directory trees.

The key question is however - what do you want?

• being able to recover earlier versions is essential when working with source code
• being able to merge such versions in text files is necessary when working on code cooperatively with others - and only source control systems can do this well
• In 99.9% of the other cases, you just want to be able to re-create a single ground-truth version of all your data after a disaster, and keep that backup copy as current as possible.

These are not the same requirements, especially the volume of data will differ.

And also, while you might to want or need to go patch by patch through conflicting source code tree with 10,000 different lines, I guess that absolutely nobody is willing or has time to go through a tree with 10,000 conflicting photographs and match them.

So the question back is: What is your specific use case and what exactly do you want to achieve?

For backup, one can simply rsync their entire home directory.

NFS for shared directory, maybe syncthing to have locally synced stuff, scp for sharing files?

Essentially, I use tar for backup, NFS / Samba for local file sharing, and git for syncing. (For specific cases, software like Zim wiki that stores to a git backend).

And it is not that I have not tried alternative solutions - for example, I tried the Coda file system. But blending version-controlled syncing and file distribution leads to devilishly complex corner cases and failure modes.

you want a solution for your computer? or for your phone?

if it's for your phone and it is still iOS then no, the Apple Ecosystem is closed, there are some reverse engineered offers but they tend to be patched out or not be reliable.

Maybe nextcloud? IDK I just use Borg. But Nextcloud allows that type of syncing that you describe, I think. I run a small nextcloud server for other purposes and don't use that feature.

Linux Mint + Filen