On the main screen of OBS Studios, click on the '+' icon under Sources to open up a menu. In this menu, you can select different audio and video sources, such as adding a video capture device to record video, display capture for screen recording, audio input and output capture, etc. Once you have added the different input sources, right-click on the source and go to Properties to add the relevant device, such as your webcam, audio interface, desktop audio or external USB mic, etc.