/usr/share/keyrings/

/usr/share/keyrings is a directory

man sources.list says that /usr/share/keyrings is the recommended location for keyrings managed by packages (while /etc/apt/keyrings is the recommended location for keyrings managed by the system operator).

At least on my system, the files under /usr/share/keyrings are exclusively *.gpg files.

A file under /usr/share/keyrings might be referenced in a file found in /etc/apt/sources.list.d/.