Sakyamuni is an ancient Indian and the founder of Buddhism. Tathagata is one of the ten titles of Buddha. Sakyamuni is a Buddha, but his name is Sakyamuni. That’s it, Tathagata is another name. There is no Buddha named Tathagata. There are many Buddhas. Sakyamuni Buddha is just one of these Buddhas, but he is the Buddha of our world. Sakyamuni was his title when he was alive, and Tathagata was his posthumous title after he became a Buddha.
Tathagata is one of the ten titles of Buddha, Tathagata = Buddha, so the term "Tathagata Buddha" is an irregular title, just like "Emperor", it is a tautology. Any Buddha can be called Tathagata, such as Sakyamuni Tathagata, Amitabha Tathagata, Great Sun Tathagata, Zhengfa Ming Tathagata, etc. "Tathāgata" is a free translation of the Sanskrit word Tathāgata. tatha means "such", that is, true and true, and agata means "coming". Therefore, the meaning of Tathagata is "come as it is". For example, "The Theory of Truth" says: "The Tathagata takes the path of truth to achieve enlightenment, so it is called Tathagata.
First of all, in Chinese mythology, Buddhism The supreme leader is called Tathagata Buddha, so he is naturally called Tathagata Buddha. Secondly, it is related to the true meaning of the word Tathagata. There is a dialogue between Sakyamuni and Elder Bodhi in the Diamond Sutra. This is also One of the most important Buddhist scriptures in the world talks about the true meaning of Tathagata. "Tathagata" means that the Buddha came to save mortals, which is static, while "come" is dynamic, which means after becoming a Buddha. There is no distinction between movement and stillness. The Buddha came to understand and save all sentient beings, so he is called Tathagata.
Sakyamuni is a real person in history, the founder of Buddhism, and ours. It is incorrect to say that the only Buddha in the world, what we call "Tathagata Buddha", is Chinese.