navigator.userAgent;
// Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.114 Safari/537.36
The User-Agent string for Big Sur's Safari running on Apple Silicon (ARM) still says "Mac OS X 10_16" and, curiously, claims to be "Intel":
"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_16) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0 Safari/605.1.15"
http://faisalman.github.io/ua-parser-js/