How did China’s geography both help and hinder China’s development as a country?

Questions like this cannot be answered definitively.  We cannot go back and rerun China’s history, giving it different geography, to see what happens.  Instead, we can only speculate.  One famous speculation about the impact of China’s geography comes from Jared Diamond’s book Guns, Germs, and Steel.
Diamond says that China’s geography initially helped the country.  He says (on p. 414) that China’s heartland did not have many barriers to break it up.  Its two major river valleys were easily connected.  This allowed the civilization of the north to interact with the civilization of the south.  The two civilizations could share ideas and strengthen one another.  This, Diamond says, helped China become a very strong and advanced country by the early 1400s.  At that point, it was more developed than European countries were.  Thus, its geography helped it by allowing it to become a unified civilization very quickly.
However, Diamond says that this geography then caused China to lose its lead over Europe.  Because China was so geographically interconnected, it was able to become one unified country instead of being many small countries as existed in Western Europe.  In Europe, the countries competed with one another and forced each other to develop.  In China, there was only one country and there was no competition.  This meant that China did not have to keep progressing.  It could stagnate in terms of technology without being conquered by other neighboring countries.  Because China was united and unchallenged, it did not have to get stronger.  In this way, Diamond says, its geography, which had once helped it rise, also helped make it fall.
https://www.prospectmagazine.co.uk/features/geography-biology-europe-fertile-crescent