/usr/lib/x86_64-linux-gnu/libavformat.so is in libavformat-dev 7:3.4.2-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libavformat.so.57.83.100
/usr/lib/x86_64-linux-gnu/libavformat.so.57.83.100