Obsidian and Syncthing
This'll be a short post, but I just have to share my experience here. iPhones have a ton of great software, but one underappreciated app is Notes. It's quite a nice interface, and I was looking for an alternative for quite some time until I found Obsidian.

Obsidian's a very full-featured app with an extensive plugin system that somehow still manages to look elegant. It's feature-packed if you want it, but manages to retain a mostly minimal design. I'm currently using their client on both Android and Windows. The best thing is their lack of lock-in - it stores entirely in folders and Markdown, so you can just port it over to a different system should any issues arise. However, it's not open source, which might be a dealbreaker.
Now, I've been using it on my phone for at least a year by now. However, I'd rather be able to edit the same set of notes from my computer and from my phone. A nice OSS app Syncthing seemed to handle it - it would sync files both ways over the local network. Syncthing seemed to be pretty popular, so I installed their client for both Windows and Android, connected the folder, and after a minute or two of straightforward debugging, it worked. Syncthing's GUI is also quite nice.

Now, I can edit my notes on PC and see them reflected on my phone in real time. If you choose to do so as well, there are a couple other options (for instance, if you're not on the same local network) and different plugin setups for those. Keep in mind that this current setup only works over WiFi.
Other Options
Here's some options I might use, but haven't really experimented with. I'm already connected over Tailscale, so it might work while using VPN. I'm guessing there'd be additional setup as well.
Another option I was evaluating (in favor of Syncthing) was rsync
, which is solely command-line. However, using Termux on a phone is quite painful, but totally up to you.
There's a lot of plugins out there for Google Drive, S3, and other cloud service sync. Two of the ones I found are obsdian-gdrive-sync with 700 stars and remotely-save with 5k stars. You can probably find your own as well.
Thanks for reading! Here are links to Obsidian and Syncthing.
Edit: Syncthing Relays
It has just come to my attention that Syncthing uses e2ee relays to transmit your stuff over public WiFi or cellular. No need for tailscale anymore!