many drugs were discovered, evaluated and approved without understanding at all how they worked.

iirc back in the day chemists synthesized a whole bunch of random compounds, observed their effects (in mice etc., or even the chemists tasting them!) then did clinical trials to measure safety and efficacy.

high-throughput screening of chemical libraries on in vitro assays is the modern version of this. "rational" drug design, which uses understanding of mechanisms to design chemical structures for a specific purpose, largely failed back in the '80s.