C-series is 46% carbon fiber https://www.materialstoday.com/compo...-with-cseries/

It is (along with a cutting edge wing with substantial carbon fiber parts) what gives the C-series better fuel burn. Part is it being a new optimized plane, part is the additional use of carbon fiber.

The 787 is 50% by weight composites, and the A350 is 53%. see https://en.wikipedia.org/wiki/Airbus_A350_XWB (under materials tab)

For contrast the 737 is 4-6% carbon fiber (depending on model) the MAX and A320ceo are 10-15%, the neo is slightly higher (can't find the figures I have seen earlier on it).