Writing letters of recommendation must be a fine art. Culture across fields and countries vary wildly in almost any respect, and this is likely to the case for LoR.
How/when do professors learn how to appropriately convey the value of the person they recommend?
- As they write them, by trial and error seeing which students land the position. But this would require a large number of trials, given so many factors come into play (including circumstances that have nothing to do with letter writing)
- As they read them, while sitting on admission committee. I am then almost completely oblivious to the procedures, but as above, many observation might be required before one can be confident on its ability to write a proper letter.
I've seen a few good ones that were written about me. I use similar thoughts and structures without copying and by supplying my own knowledge about the subject of the letter. In time as a professor, you will see letters written about others during your service on hiring, student selection, and tenure committees. You can borrow (without plagiarising) ideas and structures from the ones that make strong impressions on you.