That's exactly the thing about the banality of evil, any individual person working on some small subsystem or component of something in a FAANG, to them it might not immediately be apparent that what they're doing is enabling the corrosion of democracy and discourse. And enabling the monetization of peoples' attention into endlessly scrolling social media feedback loops.
It's the end result of all the work of hundreds, or thousands of 'engineers' and 'product managers' in facebook working together on their own projects, combined together in aggregate, that turn it into a monster.
You can also see the same thing in hardware and software engineers that work on some small discrete component of some piece of equipment in the defense industry. They might not agree with the total product as it's used in the real world (precision US weapons sold to Saudi Arabia and deployed in Yemen, for instance). But all that one engineer sees is the small item or subsystem that is within their scope of work.