Relevant part of debian

http://docs.frrouting.org/projects/dev-guide/en/latest/building-frr-for-debian9.html

sudo install -m 755 -o frr -g frr -d /var/log/frr
sudo install -m 755 -o frr -g frr -d /var/opt/frr
sudo install -m 775 -o frr -g frrvty -d /etc/frr
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/zebra.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/bgpd.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ospfd.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ospf6d.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/isisd.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ripd.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ripngd.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/pimd.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/ldpd.conf
sudo install -m 640 -o frr -g frr /dev/null /etc/frr/nhrpd.conf
sudo install -m 640 -o frr -g frrvty /dev/null /etc/frr/vtysh.conf

Same for ubuntu:

http://docs.frrouting.org/projects/dev-guide/en/latest/building-frr-for-ubuntu1804.html

sudo install -m 775 -o frr -g frr -d /var/log/frr
sudo install -m 775 -o frr -g frrvty -d /etc/frr
sudo install -m 640 -o frr -g frrvty tools/etc/frr/vtysh.conf /etc/frr/vtysh.conf
sudo install -m 640 -o frr -g frr tools/etc/frr/frr.conf /etc/frr/frr.conf
sudo install -m 640 -o frr -g frr tools/etc/frr/daemons.conf /etc/frr/daemons.conf
sudo install -m 640 -o frr -g frr tools/etc/frr/daemons /etc/frr/daemons

I tried to configure ubuntu by adding ospfd.conf and it did not have any effect - I had to add 'router ospfd' to frr.conf to make it work.

I am new and I am trying to understand how it works.

Thanks,

Aleksey