Buddhism to spread around Asia, China was the main of the source .
Firstly, Buddhism reached China from monks of India roughly 2000 years long ago during the Han Dynasty.
Buddhism was introduced to China by Buddhists who help by Silk road traders.
At first step, Confucian China was not terribly friendly to Buddhism. Later Buddhism slowly spread in China.
One of the key forces of Buddhism’s success was Daoism. To help the Chinese comprehend Buddhist concept, Buddhists gained a lexicon that made it easier to teach their tradition.
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The other key force of Buddhism’s success was some King of China emperor supported to improve Buddhism in China.
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The next step, Buddhism spread to Korea and then Japan from China.
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One of the main courses of Buddhism spread to Korea and Japan , the rich culture of China influenced the other country and the power of China emperor.
So China was the main source of Buddhism to spread around Asia.
So the population of Buddhism is more and more increased around Asia.
Large Buddhist populations live in China. China Buddhism is Mahayana Buddhism .
Something to add (补充两句）
Though it is invariably true that China is an important contributor to the spreading of Buddhism in and around Asia, it is only true if you consider Asia is made up of only China, Japan, Korea and Vietnam. Let us not forget about our South East Asia, which is also part of Asia. Buddhism came to this region from Sri Lanka (to Myanmar, Thailand, Cambodia and Laos) and from Indian travelers and traders (to Malaysia and Indonesia as evidenced by the Bujang Valley and the Borobudur). There is little or no influence from China with regard to the spreading of Buddhism in this region as far as ancient times is concerned. Btw, South East Asia Buddhism is (mainly) Theravada Buddhism.