Yes, it is not consist only USD, but also Yen, Euro etc.
But majority will be in USD.
Because world trade and export always done primary in USD denominated, except business done with Jap (they prefer Yen), and Europe. While the rest of the world generally use USD as settlement medium.
Petronas export oil, will receive USD as payment as oil is sold in USD term, so it takes the USD and exchange back to RM, BNM will have USD in the foreign currency reserves.
Foreigner buyer come in Malaysia to buy palm oil in Malaysia, they will bring USD to BNM to exchange to RM then pay the palm oil company.
Corporate borrowed from overseas in USD and bring back to Malaysia for their corporate funding, then the USD will go to BNM.
So, USD is the ultimate important currency in the foreign currency reserves coffer.
How about with China? I thought at one time, Malaysia is suppose to have direct RMB/MYR settlement exchange center and skip USD altogether therefore should not have any detrimental effect. But, recent Yuan move (intentional devaluation against greenbacks) seems to hurt Ringgit more.