Cunning redirect?

Question

Search Engine Optimization

There is a site conditionally https://example.com when you go to it from a search engine such as google or yandex, a redirect occurs to the https://test.example.com subdomain, but if you enter https://example.com yourself in the address bar then there is no redirect, but an empty page appears, Roskomnadzor naturally will not block the main domain)) How is this implemented please tell me?

Answer 1

View headers, more specifically https://en.wikipedia.org/wiki/HTTP_referer