When should I use 兆 instead of 万亿?
兆 was used in ancient China to represent trillion, it is no longer used in mainland China, but still used in Taiwan, it uniquely represents the mega prefix in the International System of Units currently. But in Taiwan, 一兆 means one trillion, while 兆赫兹 means MHz (megahertz). The largest number that can be expressed in a single Chinese character is 亿 in mainland China, to express larger numbers, use the phrases like 万亿 or 亿亿. Of course, there are also Chinese characters that represent numbers larger than 亿 (100 million), but such a large number is not used generally, and nobody use them, except the ancient literature. You may learn them here.