Earlier quoted context omitted.
Syncthing doesn't do smart sync. The library of stuff I want available is larger than can fit on my phone.
Syncthing has ignore files which can be used to selectively not sync certain files or folders per device. I'm not familiar with Dropbox's smart sync, so if it's like a transparent on-demand sync then no, Syncthing doesn't have that. Maybe it's possible to implement that as an extra app though, without even touching Syncthing. Android seems to have some concept of virtual file systems provided by apps so perhaps one c…
In Dropbox, if I want a file I haven't synced yet, I just browse to it and open it. I can also mark folders as available offline.