Stream via graphics card or CPU?

Specs:

Ryzen 9 3900x
GTX 1060 6 GB

Streaming on Twitch at 720p and 60 FPS is provided.

Games: Warzone, Rainbow Six Siege, Star Wars Battlefront

I ask because the 1060 is not the Ultra Graka.

Via the CPU. Since you have more cores with the Ryzen and you need the graphics card for the game calculation, since a game usually never uses all the cores in a Ryzen. Well, I don't know of any.

Streaming via NVENC is a problem with Twitch anyway (bit rate limit), so use your CPU, which is hardly used in games anyway.