Question about FiiO K3

Yes, it works. I tested it with the Mate 20 Pro.
About the battery: I ran the K3 at full volume with the bass and gain switch active (= maximum consumption). The Mate 20 Pro was at about 60% volume, the screen was on but at the lowest brightness level.
--> 1% of the battery lasted 4 minutes with these settings.

I have not tested it myself, but I would think that the smartphone does not supply enough power for the Fiio..., the proof of the pudding is in the eating, but be careful, the packaging is very difficult to open without damaging it so that you can send it back again.