OBS Virtual Camera: Start OBS → Tools → Start Virtual Camera, then click Start Camera below. System audio from OBS will be mixed with your mic automatically if OBS is configured to capture it, or add your mic track separately via the mic toggle.

Screen share: Click Start Screen — your browser will let you pick a window, tab, or entire screen. Enable "Share tab audio" or "Share system audio" in the picker to capture screen audio. Your mic will be mixed in automatically.

Screen + Camera: Your screen video is the main feed; camera is composited as a picture-in-picture overlay on the server side. Screen audio and mic are both mixed.