You can absolutely sync your vault without a paid subscription. Simply save it within your OneDrive or Google Drive folder. Alternatively, you could use Syncthing if you prefer a self-hosted solution.
You can, but _you_ need to figure out how to do it.
If don't know how or can't be bothered, you can pay for Obsidian Sync - which Just Works.
I tried to roll my own syncing with syncthing and iCloud and Dropbox. In the end I spent so much time debugging and dealing with files clobbering each other mid-sync I figured out $4/month to support a project I use daily isn't too much.
Zero problems since and I use Obsidian regularly on 4 different devices.
Seconding this. I use Obsidian and Obsidian Sync for personal stuff, but my employer doesn't allow Sync, so I use Obsidian very happily as a standalone on my work computer. The work vault simply never gets exposed to the outside world (we don't allow USB memory devices either).
Otherwise the free Obsidian has everything, and there are other plugins for self hosted syncing that don't use the paid service.