let's please not pretend this is something you have to invest hours of your day into
It is though, precisely because, as you say, there are many many sub-companies for nestle and coca cola. They are near-monopolies owning large swaths of products making it difficult to identify alternatives, and once you identify true alternatives, you have to determine whether those alternatives are really "more ethical" whatever that means. It's truly a waste of time.
Boycotts are different because the demands are clearly vocalized and efforts are coordinated, making for a bigger impact. There's also the carrot of "if you do xyz specific actionable thing, we will come back and start buying your products again." Boycotts also don't require the same level of time consuming scrutiny on an individual level. You look at the list of brands and you avoid those brands. Easy.
Based on your other comments, your proposed solution to iPhones is Fairphones and Shiftphones, which are not only more expensive than lower-end iPhones, but also typically only available in Europe.