How do i check which debian package is associated to a file?

[pangolinsos]

Man page for dpkg.

Nginx was install originally via the nginx_1.14.1-1~bionic_amd64.deb package

$ dpkg -S /usr/sbin/nginx
nginx: /usr/sbin/nginx

Note: If the file is not in the package manager cache, installed without a *.deb package, it will not be listed.