Live Streaming & Premieres
OBS Studio
Also called: OBS, Open Broadcaster Software
OBS Studio runs on Windows, macOS and Linux and costs nothing, which is why it turns up in almost every streaming setup. You build scenes out of sources, a camera, a game window, a browser overlay, images, audio devices, then switch between them live while it encodes and sends the stream to YouTube, Twitch or several destinations at once.
Plugins cover most of what it will not do by default. It also records locally at higher quality than it streams, which makes it worth having even if you never go live. The learning curve is real and almost all of it lands in the first setup.