Valve has long been preparing its software stack for the arrival of both the Steam Machine and the upcoming Steam Frame, and the latest update sees the gaming giant add a long list of fixes, QoL improvements, and features to SteamVR as well as changes to the developer back-end in order to prepare fo...
This image seems to explain it.
Proton is the windows compatibility layer. Lepton is the android compatibility layer. FEX translates x86 (most desktop computers) applications to run on ARM (most mobile devices).