Syncthing is a tool for syncing files and folders between computers without the need for a central server or host. I've been using it to replace what I used to use NextCloud and/or OwnCloud for.
Somewhat like .gitignore with a slightly different syntax; .stignore tells Syncthing which files you don't want to keep synced on that particular machine.
A quirk seems to be that the * needs to be last?
In .gitignore I often start by ignoring ever thing, then adding the few things I want to include:
* images/ index.html
But to do the same in .stignore I have to:
images/ index.html *
On Android I use what is called 'Syncthing-Fork' on F-Droid. It adds some useful features over the official Android version.
I've had this off-again on-again problem where a particular folder won't sync., showing the error message 'Error (folder marker missing)'. Once this was solved just because syncthing-fork was out of date, but today I had to install Simple File Manager Pro from F-Droid (as they recommend), and then go to the synced. folder and manually create the file .stfolder. This is supposed to happen automatically on upgrading.