Whenever I read the biographies of natural philosophers or scientists of Europe in 17th to 19th century (mainly Germany, England, ...) it is often written that they learn Greek and Latin in their youth.
Here are my questions:
1- Why did they learn Greek to begin with? Did they do so to be able to read works of Archimedes, Euclid or Plato in the original text? If so, the Greek language they learned differed from the language used in Greece (under Ottoman) of those ages.
2- In their biographies, when it is written that they *learned* Greek, do they mean that they became fluent in it like a modern EU C2 level mastery? Or was it like learning things like Penta Octo and word roots?
They learned Ancient Greek, which is mostly Attic Greek (Athenian; Plato, Euripides, Thucydides, etc) but versatile enough to also cover Ionic (like Herodotos) and Homeric (Homer). The ancient variations are minor, once you get the hang of them. Your scientists did not, presumably, learn what was at that time "modern" Greek, the language spoken on Greek lands in that time. Ancient and "modern" Greek are related, but not close enough to be feasible, and your scientists wanted to read the classic texts, as you mentioned.
They became fluent. They could read ancient texts, even if it was a bit slow going at times (and generally still is for us Classicists today). They could probably write a bit in ancient Greek as well, though that is and was a bit more advanced than simply reading. They wrote all of their scholarship in Latin, however.
This was the general curriculum of all educated persons in the 17th to 19th century, not just scientists. Some had more proficiency than others with ancient languages (then as now), but all were exposed to both Latin and Greek and expected to master them, at least on the level of reading (in Greek) and reading/writing (in Latin).