Some people may see animated confetti, fireworks, or balloons during a Studio session if a participant is using an Apple device: a Mac, iPhone, or iPad. This is not a Riverside feature, but rather a feature from Apple called Reactions.
If a participant has Reactions enabled on their Apple device, their video feed shows these 3D effects when they make specific hand gestures, like a thumbs-up or a peace sign.
For more details on Reactions, please see this guide from Apple.
On a Mac
After Riverside starts using your camera, click the Camera icon in the system's menu bar, then click Reactions to them off or on.
On an iPhone or iPad
After the Riverside app starts using your camera, swipe down from the top of the screen to open the iOS Control Center. Tap Video Effects, then Reactions to turn them off or on.