Is Dashcam legal in US?

Could you clarify for me if Dashcam usage is considered legal within the United States? I've heard differing opinions and am wondering if there are any specific laws or regulations in place that govern the use of Dashcams in vehicles. I'm particularly interested in whether there are any restrictions on recording audio or video footage, and if so, what those might be. Additionally, I'd like to know if there are any privacy concerns that drivers should be aware of when using Dashcams, and how to ensure that I'm complying with all relevant laws and regulations. Thank you for your help in clarifying this matter.