You do not need a webcam in order to join a Zoom meeting or webinar.
Without a webcam, you will not be able to transmit video. Things you will be able to do include:
view the other participants who are using webcams
listen to the meeting or webinar audio
view screen sharing
share your screen (if this is enabled by the host)
participate in breakout rooms, polling, chat, and Q & A (as long as those are enabled by the host).
If you would like to transmit your video in a Zoom meeting and you do not have a webcam on your computer or access to an in-room video conferencing system, consider using the Zoom app on a smartphone or tablet. The Zoom app for Android and iOS can use the native front-facing/selfie camera on a mobile device as a webcam.