Why? Chinese are a peaceful people and don't fight wars. We rather trade, make money and build infrastructure!
All these ships and airplanes are to give invaders something to think about!
If you are not going to invade China, you don't need to know if they are good or not
Then who fought the Pangong Tso battle and lost a bunch of lives? Mongolians?
Who is threatening to invade Taiwan? Vietnamese?
1. First of all invading China is pretty easy - there is a bunch of disgruntled nations all around you, with many like Japan, Korea, and India having superior tech in several areas - so you'd be taking on multiple giants.
2. PLA has very llittle experience doing joint training exercises - so we can expect very behind-the-curve war tactics.
3. One child policy means male members are a lot more valuable and therefore cannot die. Plus, like most East Asian society, Chinese are a patriarchal society.
4. Occupied Tibet and Xinjiang are very large areas - low level rebel attacks will tie down the entire PLA here, much like Balochistan ties down the Pak Army due to the sparsely populated, geographical spread, Not to mention the cost of warfare goes up tremendously.
5. Most likely the Quad blocks Malacca strait and the Gulf shipping lanes, depriving China of much needed energy supplies. China being an export-based economy, is going to suffer tremendously.
6. China is food insecure - there is not enough land to grow staple crops to eat plus seasonal floods devastate food all the time. If countries stop exporting food to China or make it pricier, many Chinese will starve to death.
7. Very little livable land area - the livable portion of China is less than the size of India - and even less cultivable area. This means effective annihilation of Chinese cities needs to happen only on a smaller, dense area, just about the size of Indo-CHina countries put together. No one has to bomb the vast unlivable areas of China. So the cost of warfare is actually smaller for the nations that bomb China. But cost of warfare is huge for China as you have to bomb the whole of India, ASEAN, Japan, far away Australia, S. Korea etc to establish deterrence.
8. The livable areas of China has no access to any emergency resources as it borders Siberia - another wasteland to the north where nothing grows. And North Korea has no strategic depth or resources to help.
So, let's see livable area of China is surrounded by wastelands where nothing grows to the north and west and by hostile powers to the east and south. At the end of the day hemmed in, small dense GDP area presents a huge threat for the PLA due to a lack of strategic depth - something India's neighbor Pakshaitan also faces as their livable land area is a small sliver along the Indus and that too to the east of the river where all their major cities from Islamabad to Lahore to Karachi can be hit just with Indian artillery. Their most economy producing city Lahore can be hit with just with small arms - they are right on our border. Same goes for China - Taiwan and Japan and Korea can hit top tier Chinese cities with low cost, long range artillery, much like India can hit Chengdu and Kunming from our northeast.
So, you see China's 95% GDP producing small area is screwed. So, you better fly those J20s and test them in a mock battle, as you need to conduct long range, stealthy operations to defeat your enemies. Your survival is at stake. Get on it.
Either that or disband the PLA.