I thought that things like onFormSubmit and onOpen trigger automatically at those events just by virtue of having those function names. But that doesn't seem to be happening. Did I understand that wrong? And if so, what's the point of these reserved functions? Can't any function be set up to trigger when a form is submitted or what have you? I'm confused.
There are two types of triggers:
I thought that things like onFormSubmit and onOpen trigger automatically at those events just by virtue of having those function names.
That is correct. Things that run by virtue of the function name are simple triggers and they have multiple restrictions-like they can't run more than 30 seconds. However, there's no onFormSubmit simple trigger(see below)
But that doesn't seem to be happening.
You need to figure out why that "seems" to be so or what made you come to that conclusion. The executions would show why a function failed to execute, when they are triggered.
Did I understand that wrong?
No. You understood correctly.
Can't any function be set up to trigger when a form is submitted or what have you?
Yes, Triggers set up this way would be installable triggers. The restrictions here are pretty lax than the simple triggers - like they can run for 6 minutes or if you have workspace account, 30 minutes.
https://developers.google.com/apps-script/add-ons/concepts/editor-triggers
Do note that Form submit is only available as a installable trigger. See the table in the above link to know what type(s) of triggers are available for which application.