Sorry for the late reply, was testing if the new version finally worked for android 10 (it does).
Well, first thing you want to do is to configure syncthing, install it on the devices you want to use, the way it works is you set up a folder to share and then you do it with the "paired" or trusted devices, or at least that's how I do it, you do a empty folder of any name and wherever you want, just remember its path for that's where you are going to be saving everything, it is essentially local storage, but the "sync" part is done by syncthing peer to peer.
I would use the device in which I have the most notes as the main for setting up syncthing and Joplin in the case of thing going wrong, since you would lost the least in that way if the failsafe pops up, it is also advisable to do a backup of all your notes, so you can then merge them into one folder if it does goes wrong and you don't lose anything.
In syncthing go to devices or Add a remote device, every device has a unique ID, copy that either by a safe channel like a personal note on another app like bitwarden or signal and then copy it to the other device, also put a "Name" no the device so you can easily identify it, I haven't seen this on the windows client, but in the android one it has like a discovery option, so you can easily select the device, after that you can inmediatly share folders but I like to do it separatedly.
With your folder created using any file manager you select the folder or just copy the path on syncthing through "Add folder", also needs an idenifier which is the folder ID and a Label which would be the Name, the folder ID is the same for all devices synced, the Name varies in every device if you want. In the sharing tab we select the devices we want that folder to be shared with. In fole versioning I use simple file versioning on the main device or "seed", so I have like yet another backup. No ignored patterns. And on the advanced tab check watch for changes since that would pop up synchronization every time we sync on Joplin, set the folder to be Send and Recieve on all devices, and everything else is not really much impactful.
The folder is still empty, all this has just been syncthing configuration.
On Joplin go to tools -> options -> Syncronization
And set the path of your empty folder, and modify any options you desire, click apply and ok and then start the syncrhonization. Although before doing it you may want to set up end to end encription so even if syncthing fails (which is higly unlikely since is the safest and most private method) your notes would be inaccesible if the password set on the configuration is unknown.
Right now Joplin is synced locally, syncthing is used as client for synchronization, you have to follow the same steps in every device for Joplin, but once is done you don't have to touch it again.
As a note, becasue of how syncthing works, the sync will only occur if there is another device on, let's say you are on the street and you save a note in Joplin, it will save it locally, but since there is no other peer on, that information cannot be shared nor updated, but it will sync everything once you turn on your other device, it doesn't pass through any server and is direct, if you do have your computer on and do a change but are not on the same LAN the connection and changes will be made using TLS, so it is encrypted as well as it passes through servers in the internet.
If you have doubts or anything you can contact me through here or:
@arendia:matrix.org on [Matrix] Element