SUMMARY Yesterday I set up a Neon VM and the first thing I noticed was that single clicks opened folders in dolphin. To fix this, I looked though dolphin's settings, but to no avail. Then I remembered that this was a system setting, and managed to find it in "Workspace behavior/General behavior/clicking files or folders". It's reasonable to have it there, since it affects items on the desktop too, which aren't handled by dolphin itself, but I would suggest mirroring the setting in dolphin as well, since that is the more likely place someone will look for it, I believe.
You know what, the front page of system settings is probably so hard to miss, it's a miracle I managed to do it anyway. Never mind.