Do UAVs use ground-based navaids? When I Googled it seems it is not common to use Vortac like systems from UAVs as a backup to GNSS outages, like in manned aircraft. And also I haven't seen any equipment designed for just UAV platforms, it's all designed for in-cockpit use.

Why do UAVs not use ground-based navaids?
For UAS, the aircraft may be below the service volume of a VORTAC most of the time.  Furthermore in the US sUAS are normally line of sight to the pilot so in theory if GNSS is unavailable, the pilot will control visually.  Finally the variability and uncertainty of VORTAC is high enough so as to make it impractical for UAS stability, drift and position control