As a (BrE) footnote:
1. The Yangtze is the longest river in China, that flows eastward from Tibet to the China Sea.
— this non-restrictive use of "that" may be found in older texts, but is not so common these days. It is unpopular with ESL examiners.
2. The Yangtze is the longest river in China that flows eastward from Tibet to the China Sea.
— this (restrictive) use implies that other rivers also flow eastward from Tibet to the China Sea; but that of all those rivers, the Yangtze is the longest.
3. The Yangtze is the longest river in China which flows eastward from Tibet to the China Sea.
— as #2. Also unpopular with ESL examiners.
4. The Yangtze is the longest river in China, which flows eastward from Tibet to the China Sea.
— this use is slightly unidiomatic, but is sometimes encountered. More usual:
5. The Yangtze is the longest river in China, and flows eastward from Tibet to the China Sea.
— or if you want to retain the non-restrictive relative pronoun:
6. The Yangtze, which flows eastward from Tibet to the China Sea, is the longest river in China.
MrP