以…为主【yǐ…wéi zhǔ】

以…为主 says what the main part of something is: "mainly", "consisting mostly of", "predominantly". The main element goes between 以 and 为主, and the whole frame is the predicate of the sentence. 为 reads 为wéi here, "to be". It is neutral to written – descriptions, reports, weather forecasts.

What goes inside can be a noun or a verb phrase.

为主，为辅

The secondary element can follow with 为辅 "as a supplement".

Don't add 主要

为主 already means "mainly". Adding 主要 says it twice.

• × 我们的饮食主要以米饭为主 → 我们的饮食以米饭为主 / 我们主要吃米饭

The wider 以…为… frame, "take X as Y", has its own point.