Not sure, googling often gives bigger and more polycount assets (animated or not, single vertice groups or multiples) that requires so much conversions before it could work in game. and these pieces were made in real scale and not 'Disney scale' (Just think of Frozen, where the sequel is on screen as we speak)
For Units it is harder, although you can have joy with vehicles etc. The Sketchup 3D Warehouse is the best place for free models these days. If you're prepared to pay there are sights like Turbosquid.
Generally for human units it will be simpler in the long run to adapt them by using parts for different existing Civ VI units otherwise the style will be out of place. This is how most of the units in Steel & Thunder were made.
If you have no familiarity with Blender or 3D graphics there will be a lot to learn. I've been working with 3D graphics for more than 10 years and I've enjoyed it. It's really up to you. If you expect it to be easy and zero frustration then I'd say probably don't bother. If you are a patient and determined person who enjoys learning new things then give it a whirl.