Update because you are using varnish, nginx will neither see nor respond to all of your requests, so nginx will not have an accurate count of the number of page views. In this case, I typically trust GA to have a reasonably good measure of your traffic. If you would like to use nginx to verify which of your analytics frameworks has a more correct answer, you should turn off varnish. Varnish typically doesn’t have logging to a file since it would impact performance (it logs to a shared memory segment). But, if you do want to set up varnish logging to a file, you would want to use something like varnishcsa.
nginx will log all requests that nginx responds to in the access log so long as you have configured it correctly. Other factors such as
logrotate may affect whether all entries show up in your access.log file. In addition, pre-nginx caching (through something like cloudflare or varnish) may affect whether or not the request is responded to by nginx.