syncing a folder between mac and Iphone

Install Syncthing on your Mac and Synctrain on your iPhone, and connect them together. Share the folder and keep Synctrain open on the iPhone screen until the initial synchronization task is complete. When you want to make changes, keep Synctrain open on the iPhone. I wouldn't trust background app activity, as it could be terminated at any time by the operating system.

I haven't used it, haven't needed to yet, but pretty much whenever you ask about this, anybody who knows anything mentions it. In fact I need to dig into it to sync things between my Mac and my NAS at some point.

Syncthing is really cool! I started using it about a month ago, and now wonder how I got along without it for all these years. The major thing to know about Syncthing is that it isn't meant to be a backup tool. Data syncs between devices that are online and can see each other. Having said that, you can enable file version history on a folder by folder basis on each device, but it shouldn't be relied upon as a robust or your only backup solution. If you really want to use it as such, set one of your devices to be receive only rather than send and receive, so it can only accept changes from other devices without pushing any back to peers. You can also create send only folders which do the opposite.

https://apps.apple.com/us/app/synctrain/id6553985316

Have you tried modifying the country code? Change the /us/ section of the link to the two-letter code for your country and see if it loads.

There's another Syncthing app you can try, but this one requires an in-app purchase, and I don't know how accessible it is.
https://apps.apple.com/us/app/m%C3%B6bius-sync/id1539203216

You could always try using a VPN or different Apple account based in the United States to get around these ridiculous artificial geo-restrictions.

First, make sure your devices are connected. Open Synctrain and leave it there. Make sure the screen doesn't lock. IN the Syncthing web interface on your Mac, go to the remote devices heading and select to add a device. If you kept local discovery enabled, it should find your iPhone. Select it here. On the iPhone, go to the devices tab and find the ID for your Mac. Confirm you want to add it. Once you do, it should say connected unused on your Mac to indicate the devices are connected, but you haven't shared a folder.
Next, locate the folder you want to share in the Finder. Open the context menu and move to the copy option, but don't do anything yet. Press and hold the Option key so it changes to the copy full path option. Press enter while continuing to hold Option, and the full folder path will be copied to the clipboard.
Go back to the Syncthing web interface on the Mac and select the add folder button under the folders heading. ON the general tab, give the folder a friendly name like My Video Collection. Leave the folder ID box alone unless you want to make it simpler. This ID must match on all devices you share the folder with. Paste the full path to the folder in the path box by pressing Command-v. Find the sharing link and select it. This will give you a list of checkboxes to specify which devices you want to share the folder with. Select the iPhone, and then select the save button.
Finally, you need to accept the folder on the iPhone. Go to the folders tab and it should automatically appear under discovered folders. Select it, then make sure you change the option from sync selected files to sync all files unless you want specific files to sync. Select the done button, and you should be good to go. Keep the iPhone app running on the screen until the sync is finished.
If you find this is still confusing, I'll record a podcast and submit it to the website so it helps other people in the future.