In my experience, sometimes your employer blatantly lies to you about what you're making and how it'll be used. I was once recruited to work on a software installer which could build and sign dynamic collections of software which was meant to be used to conveniently install several packages at once. Like, here's a set of handy tools for X task, here are the default apps we install on machines for QA people, here is our suite of apps for whatever. It seemed to have genuine utility because it could pull data in real time to ensure it was all patched and current and so on. That could be great for getting new machines up and running quickly. Several options exist for this use case today, but didn't then as far as I recall. This was on Windows.
Ultimately it was only used to install malware in the form of browser extensions, typically disguised as an installer for some useful piece of software like Adobe Acrobat. It would guide you through installing some 500 year old version of Acrobat and sneakily unload the rest of the garbage for which we would be paid, I don't know, 25 cents to a couple dollars per install. Sneaking Chrome onto people's machines was great money for a while. At one point we were running numbers of around $150k CAD per day just dumping trash into unsuspecting people's computers.
At no point in the development of that technology were we told it was going to ruin countless thousands of people's browsers or internet experiences in general. For quite a while the CEO played a game with me where I'd find bad actors on the network and report them to him. He'd thank me and assure me they were on top of figuring out who was behind it. Eventually I figured out that the accounts were in fact his. They let me go shortly after that with generous severance.
I don't miss anything about ad tech. It was such a disheartening introduction to the software world. It's really the armpit and asshole of tech, all at once.