China has only possessed modern weaponry for the last decade or so, and during that time they have never engaged in combat so their command and control infrastructure is completely untested. Japan’s military is expressly limited to defensive capabilities so they lack projection, but their military is highly equipped, well trained, and very sophisticated.
Ever see the security guards outside of banks in China, loading money onto and off of armored trucks? The ones with their helmets unstrapped, shoelaces untied, shirt unbuttoned, and awkwardly holding USSR-era beat up shotguns like they’ve received no training at all? That’s how I picture the Chinese army: untested.
China has had to build most of it’s armaments while Japan has had access to foreign technology which is vastly superior and is battle tested. Meanwhile China utilizes spies (some based in Chengdu) and reverse engineers foreign technology to power its modern weaponry. When these weapons are developed (by foreign nations) though a counter measure is simultaneously developed expressly for the possibility of theft – an enemy stealing the weapon and using it against you.