My home and office computers are synchronized via the ownCloud server located at my workplace. This synchronization takes place via an TLS encrypted connection with an A+ Qualys rating. In addition, I encrypt files containing sensitive information prior to their transfer on an individual basis. Since all systems accessing this cloud folder are driven by an operating system (OS) that I trust and largely control (Archlinux), I feel very comfortable regarding the security and privacy of my data. To keep this warm and cozy feeling, I wouldn't give devices with an OS beyond my control (such as Windows, MacOS, Android, iOS) access to this folder.
Now, I fully appreciate that even a hardened desktop Linux would have difficulties to compete with the level of security offered by an up-to-date Android – with “security” being defined here in the conventional context with respect to a potential third-party adversary. But concerning the privacy of my data, and thus mine, the threat of an overly nosy first party is much more palpable. Actually, I shouldn't call it a threat. It's in fact a promise.
How, then, am I supposed to transfer or even better synchronize data from and with the newest member of my gadget zoo? Since I've acquired this gadget as a two-factor authentication for my banks, I cannot simply root it and install LineageOS without any Google services. Therefore, I won't trust the device beyond its specialized purpose and I won't give it access to my cloud folder.
But that's actually not a big deal in this case. Because of its specific function as a two-factor authenticator for my banks and several other services, the phone will remain stationary. Hence, I need to synchronize within my LAN, but not outside of it. When looking for apps that would be suitable for this task, I was initially attracted by those appealing to the nerd in me, such as, for example, croc installed (pkg install croc) and running within termux, or juiceSSH. In the end, these apps turned out to be fun for a limited time, but too tedious for everyday use. I'm very fond of controlling computers with a keyboard, but for using termux efficiently, you'd need the eyes of an eagle, the fingers of an elf and the dexterity of a spider monkey.
For ordinary human beings, syncthing is the tool of choice. It's available on f-droid, easy to set up on all participating systems, and works reliably without manual intervention. In my case, I've simply created a folder (~/androidshare) on my desktop that automatically receives all files from my phone that may be worth to keep, including the backups of the andOTP and keepassDX databases and all photographs of my cats. 😍