Franky,
I edit medical research papers for about a dozen medical journals, mostly concerning biological chemistry and molecular biology, some concerning neuroscience, infectious diseases, and plant biology. I'm self-employed as an "independent contractor", so I work from home and set my own hours, which is sometimes good and sometimes not.
Basically, the researchers submit their papers for publication, and after a bunch of peer reviews to make sure that the research is sound and valid, the journal accepts the article for publication. It goes through some preliminary stuff to make sure it's accepted by the publishing/printing softwares etc., and then it comes to me.
My job is to make sure it's grammatically correct, first of all. But also, I check and correct scientific nomenclature for genes/proteins/enzymes etc., check/correct taxonomy and genus/species nomenclature, virus classifications, disease names, all that. I have to know, for example, that AB refers to a blood group (usually) and that Ab usually means antibody. I need a working knowledge of molecular biology and organic chemistry to know the difference between, for example, covalent bonds and ionic bonds, and that they are written and expressed in different formats. Stuff like that. If I have any questions (is XYz really a protein? I think it's a gene, so should it be italicized in the text?) I talk to one of the authors and get clarification. They are at research institutions worldwide, so one day I'm e-mailing Dr. X in Shanghai working on a cure for pancreatic cancer and the next I talk to Dr. Y in Brazil working on Zika virus.
For certain Journals, I have to insert coding so that it prints correctly. For example, every time a reference is called out in the text, it has to be surrounded be the right coding: <zref>Ref. 13<zrefx> etc. I also make sure that figures are labeled correctly and that they are cited in the right places in the text, and that there are no duplicate references. There's more... it's not difficult, just time-consuming and tedious.
Publishers like the ones I contract with usually only hire people who have a medical or science background of some sort; it would be difficult for someone without medical knowledge or a background in molecular biology or organic chemistry to learn all that. It pays well, but, being self-employed, it is what you make it. You can earn as much or as little depending on how motivated you are. I have been with my two publishers a while, 10 years for the one, and going on 4 years with the other. They renew my contract annually.