Should I add add_filter () before apply_filters () in Wordpress?

I am trying to understand a Wordpress plugin, for example:

apply_filters ('gettext', $ translations-> translate ($ text), $ text, $ domain);

I am looking for all the codes in Wordpress, I can not find:

add_filter ('gettext', ....);

Why is there no add_filter for this plugin? Or am I missing something? The same as:

do_action ('wp_loaded');

I can not find:

add_action ('wp_loaded', ....);