Translation:He makes a salad with beans, carrots, and tomatoes.
The verb "faire" means "to do" or "to make" depending on context. They coopted it for use when talking about the weather but it isn't only used then. It's used in many different ways.
Why not "it's a salad with beans,some carrots and tomatoes " What's wrong with this
That would be "c'est une salad avec...". If you want to say "he is making a salad with..." you would use "il fait une..."
In French, you don't put the Oxford comma (a comma before the last item in a series). It's one of the subtler differences between French and English punctuation usage. If you're interested in learning more about punctuation in French, see this article: https://www.thoughtco.com/how-to-use-french-punctuation-4086509
He makes salad or he makes salad; They both seem to translate as: Il fait une salade. So should we just guess which is being made?
I'm not seeing the difference between the two English sentences you wrote. Can you elaborate?