Do some research into
Syncthing. And also SyncTrayzor
v2 (also available as a portable). I use SyncTrayzor
v2 Portable myself to sync files between my Desktop PC and my laptop. The laptop is mostly just sitting in a cupboard and I only switch it on once a month to do updates. The syncing to the laptop gets done via the Wi-Fi of my router. My Desktop PC is connected to the router via an Ethernet cable, though it could just as soon be connected by Wi-Fi and that would make no difference.
I suppose you could conceptualise Syncthing somewhat by thinking of it being a bit like OneDrive (which I have never used) without some middle-man having access to your own files/data.
The only difficulty I ever came across with Syncthing/SyncTrayzor
v2 is if a file is altered/changed on both computers then Syncthing reports a conflict for those two versions of the file and you have to manually choose which file is the 'master', which can be a bit confusing. However, with practice and understanding of what Syncthing/SyncTrayzor
v2 are doing you will learn how to keep conflicts to a minimum. And with practice it becomes easier to identify which file version you want to keep.
If you are using SyncTrazor
v2 Portable then it automatically updates itself and Syncthing too. And when you download SyncTrayzor
v2 Portable it already contains the latest implementation of Syncthing. To use it both computers need to be switched on and each computer needs to have a running implementation of Syncthing/SyncTrayzor
v2. However, you don't have to have both computers always running at the same time — once you do have both computers running at the same time Syncthing/Synctrayzor
v2 will just pickup the systems when they were last 'linked'/connected.
P.S. Syncthing also allows connecting two or more machines over the internet, so the machines can be very remote from each other. However, I've never used that facility so can't comment further.