Out of curiosity I used renshuu's dictionary to look up how you would say alchemy in Japanese. I came across 錬金術（れんきんじゅつ）as my desired result for a general term. However, I also came across 錬成（れんせい）as another option, but clarified with the note (modern fiction). Mind you, it's not the main sense of the word.
I looked on Wiktionary and found the same clarifier, and I tried to look into it myself, but I couldn't find any explanation as to why 錬成 has this clarifier about modern fiction.
Is it used primarily in modern fiction? Is it more useful in a modern context? Does it indicate a fictional version of alchemy, and 錬金術 is for the historical version? Which one would I want to use more in a pop culture context?
Any explanation or even just direction would help.
錬成 doesn't mean "Alchemy", it points to the process of "alchemical transformation". So you would 錬成 iron into gold, for example.
錬金術 is the actual terms for "Alchemy". You might know it from Fullmetal Alchemist (鋼の錬金術師), assuming you watch anime.
Edit: I assume it's classified as "modern fiction" because it's not real-world chemistry. Practically the only place you'd see it used nowadays is fiction. Don't quote me on that.
PS: I assume you already know the difference between alchemy and chemistry, so I won't go into that.
I did notice the difference in precise meaning! 錬成 seems to be more particularly an action (especially chrysopoeia) rather than 錬金術 which is the broad concept in itself. I was mainly curious if that meaning can be used anywhere or if there would be limitations in its usage pertaining to that (modern fiction) classifier.
As far as I know, outside of actually talking about alchemy, 錬成 doesn't really come up in normal, real-world contexts. I've only ever seen it in fiction, so the "modern fiction" tag checks out. Keep in mind that I don't know enough about contemporary scientific subjects in Japanese.
For example, if you look at [Weblio], it's just quoting Arifureta (ありふれた職業で世界最強) for that sense of the word.
Edit: Strictly talking about the second meaning of 錬成, the first ("raining, drilling, strengthening") does get used.
