USB 3.0 to HDMI adapters are typically not "plug-and-play" and require specific drivers to function because they act as external graphics cards Finding the Right Driver
Due to Apple's tightened security, you may need to boot into Recovery Mode (Apple Silicon: hold power button; Intel: Cmd+R) and reduce security settings to allow the driver to load. This is a major hassle; many professionals avoid USB 3 to HDMI on Mac entirely and use native USB-C/Thunderbolt to HDMI instead. usb 3 to hdmi driver
Most adapters use one of three major chipsets. Identifying yours is the first step: DisplayLink USB-C to HDMI (Passive Cable): Requires NO driver
The adapter might only appear as a "Removable Drive" containing a basic manual or an outdated installer. Major Chipset Drivers and compatibility issues.