Depending on where a cam site operates, they may be legally required to record all video transmitted through their site. Never known of video of a regular member being used for advertising or made available to anyone other than law enforcement.
Models have video of them used in all sorts of ways though. It varies by site. Could be shared with paying viewers, or used in advertising. There are usually sections in the contract a model has to agree to that include how video may be used. Some sites allow all models to opt out of their recordings being publicly shared, some allow only their top models to opt out, and some allow none to opt out. Video usually belongs to the cam site long after the model quits too. See ads & video archives on some sites featuring models who haven't worked there in years.