How do you self-teach yourself about a field you don't know about, specifically for STEM?
Would they read a review from major peer-reviewed journals, such as Science or Nature, and then dig deeper from there?


Yes.


  How would they get teach themselves techniques, and knowledge?


Techniques can also be learned from journal articles, but it's also typical to defer to or collaborate with other researchers who are more familiar with those techniques, or to travel and visit labs. In my personal experience, several times contacts at conferences etc led to communication between labs. The lab I worked in during grad school a few times hosted grad students from other labs both inside and outside our institution that spent a few days or hours here to see how we did a particular thing, and similarly people from our lab would travel around.

When academics travel to give talks at other universities, one of the key parts of their visit is to tour other labs, which can include observing techniques used by those labs.

I'd add that this doesn't just apply to new or unfamiliar techniques, either: it's part of the normal art of improving research methodology.

More broadly, though, mid- and later-career academics don't tend to move from field to field arbitrarily (whereas it's not uncommon to make a big jump between undergrad and grad school: you simply aren't that committed to a field at that point): they usually have some adjacent knowledge that's taking them into that area, so they aren't starting completely from scratch.