After searching on WHO site, it seems that they do surveys of each countries health system, and pushing them to develop steady healthcare systems, and to adapt their practices to available medical knowledge. The pushing of WHO to develop healthcare available including to the poorest may have been what Trump disliked, after all.

The WHO seems to compensate partly for the absence of universal healthcare in the poorest countries, by direct investment.

I suspect that it is also directed towards communication and information. Like teaching the developping world the known techniques to mitigate malaria, the primary hygienic gestures's importance (like using soap to wash hands), etc. And of course the importance to have protected sex to fend off AIDS, etc. (which is an upstream battle given the religious and traditions resistance to many of these advises)