We don't suggest you use QuickTime with ezcap159 for new macOS Big Sur, you also find the reason in the below contents.
We suggest use OBS mac version with ezcap159 for macOS Big Sur.
Please download OBS from www.obsproject.com/download
This issue has been widely discussed on the Internet. Because the new big sur is in the SIP domain, those native apps do not recognize the core media plug in driver of the third party device.
The following link is a discussion on this bug on the Internet.
If you want the support of the third-party driver, you can turn off SIP. The method is as link below.
The currently known quicktime, facetime, and photo booth do not support the third party driver.
OBS, Chrome has tested it and it can be supported.