Jisho

The definition of 生, with regard to food, focuses on the uncooked or unprepared nature of food. Something that is 生 can stay 生 as long as you don't cook it, even if it starts to go bad.

The definition of 新鮮 focuses on the newness. Once some time has gone by, it will lose this, regardless of how it's handled.

So, yes, "raw" for 生 and "fresh" for 新鮮 is the best way to understand them. The fact that both words have "fresh" attached is likely only due to the fact that many raw foods are only good when fresh, and so the two words are related.

So yeah, something may be both 生 and 新鮮, but they're not interchangeable in the sense that replacing one or the other would leave the meaning exactly the same.

そうですか！ありがとうございます、リーボさん！