For Syncthing I had to add a bunch of rules to my firewall to allow the necessary connections between my PC and smartphone. And for that I had to find, install and familiarize myself with a fire wall first. And after that ensure that the fire wall service is running always. Summa summarum: it's not something that is likely to work out of the box.

The great thing about Syncthing is that once you have it set up properly it really does work. It silently does its thing in the background and I never think about it

Haven't used LocalSend yet but I imagine it's going to be much less of a pain if the traffic is all routed through the Brower.