I was wondering when Google serves content via AMP because it never happened to me. I mostly use DuckDuckGo but occasionally use Google when I'm not satisfied w

I was wondering when Google serves content via AMP because it never happened to me. I mostly use DuckDuckGo but occasionally use Google when I'm not satisfied with the results. Turns out when you use Firefox on Android, Google serves a different (older?) version of the Google search that doesn't link to AMP. Is AMP opt-in for site owners? Or is it determined by page load speed? It didn't use AMP for my website, but it also loads faster than google.com ¯\_(ツ)_/¯