/usr/lib/.... path usually for packages,daemon,services that you install or exist when you installed your os . Relevant config also may exist in /etc/..
/etc/.. path is the place where you as a administrator should be configuring
for ip forwarding you should not be touching any file on /usr/lib/…
same goes for firewalld, systemd for them /etc/firewalld/.. or /etc/systemd/…
you can directly edit the good ol /etc/sysctl.conf or preferably /etc/sysctl.d/99-sysctl.conf
the 99-sysctl.conf is symbolic link to the /etc/sysctl.conf
upon reboot or sysctl -p allows the systemd-sysctl service read the link/nonlink files if they exist if not read sysctl.conf and make necessary entry in /usr/lib/.. and/or load your defined or other variables that there exist and ofcourse giving you ip forwarding for this scenario.
in simple al custom goes in /etc/
you should not be editing /usr/lib/sysctl.d/50-default.conf
you can edit /etc/sysctl.conf
or /etc/sysctl.d/ 99-sysctl.conf
or /etc/sysctl.d/<numberdigits and nameappropriatelyaccordingtoyourpurpose>.conf
the last one is user created it dont have to be a linked file. systemd-sysctl will load the values