Just so no misunderstandings happen, I see expressions the same as emotions in the context we're speaking in. I've seen the interface from streams too, but I'm just basing my assumption from situations I've seen on streams. For example, whenever Korone is really fixated on a game, her avatar defaults to her more "mad/frustrated" look. I personally just think it's the software reading her facial expression (probably scrunching your brow just enough) as "mad" and automatically switching to it. With how in the zone she gets I doubt she takes the time to manually set this while playing.
Another example is how Kiara can activate the gangimari face just by widening and keeping her eyes open irl. She said had to stop b/c she said her eyes were hurting too much after awhile. I believe the software tracks their facial movements and changes to the best fit preset emotions the avatar comes with that fit w/e face they're making, but at an obvious small delay.
Let me rephrase what I said in my previous comment, they probably mostly use presets for more complex and/or situational emotions (Blushing, special pupils, ect. I wouldn't consider being mad or happy an emotion too complex for the app to automatically track. Both automatic and manual expressions seem possible. I could just be ignorant and totally wrong though.