VirtualCam is a plugin that you need to install, it does not come with the base OBS installation. Here's a link to the plugin page:
MOD EDIT: This version of the plugin is currently deprecated and should not be used with OBS Studio v25. Please use this version instead: https://obsproject.com/forum/resources/obs-virtualcam.949/ We on the OBS team thank CatxFish for their hard...
obsproject.com
Zoom is not an RTMP service, and so it will not show up in the services list. You need to install the OBS-VirtualCam plugin (above), select the Virtualcam output in Zoom's client as your webcam, then start the virtualcam in OBS' Tools menu or you will just get a gray screen in Zoom.