FreeBSD is started up by the program 'init'. The first thing init does when
starting multiuser mode (ie, starting the computer up for normal use) is to
run the shell script /etc/rc. By reading /etc/rc and the /etc/rc.d/ scripts,
you can learn a lot about how the system is put together, which again will
make you more confident about what happens when you do something with it.
If you have a fediverse account, you can quote this note from your own instance. Search https://mastodon.bsd.cafe/users/fbfortune/statuses/115090471940944936 on your instance and quote it. (Note that quoting is not supported in Mastodon.)