How to share/sync files between various devices

Syncthing is a very versatile tool that synchronizes various directories between devices – be it your files or app data. While it has some limitations (it might not work seamlessly with every app you'd like to) it seriously gets the job done. It's quite easy to set up and supports a wide range of OSes. I'm using it to synchronize data between two Android devices and a Windows PC (with a plan to add another linux PC):

Various files and papers

Instead of copying the files manually it's handled by Syncthing - all the files, images or memes you'd like to keep at hand irregardless on what device you've found them. Syncthing can also be used to synchronize data between people, where both sides can be behind private everchanging IP addresses.

Phone photos

While they're not exactly backed up, they're synchronized to a different device; that makes cleanup more efficient since you can start on one device and continue on another, and after that you can use a backup solution of your choice. What's important here for me is that as long as the Syncthing is running, I don't have to manually download my photos to my PC, and I get some great transfer speeds.

Joplin notes

While Joplin (a great note-taking tool!) has quite a few built-in options, Syncthing might be the simplest way to keep all your notes synchronized between devices without any cloud services or additional infrastructure.

Warning: that's a tricky one - there is an issue that's supposedly fixed, although I still need to use v. 2.5.5 to get everything to sync properly. If the latest version doesn't work for you, you may want to consider downgrading the app and disabling auto updates.

OsmAnd saved places and map data

OsmAnd is a very interesting app that i stumbled upon when searching for a privacy focused map app with an ability to record my tracks. It's focused on offline data (free version allows for download of 7 regions) and has quite a steep learning curve compared to Google Maps, but OsmAnd looks very promising.

I was able to sync it with Syncthing when I chose in settings a custom storage path on my SD card and located it in a subdirectory of Android/media - otherwise Syncthing's directory picker didn't see it (even though the app itself has full filesystem access).