The border issues between China and India are a consequence of the expansion of the British empire. India simply inherited these borders when it became independent.

China is not really expansionist. What it does is to formally refuse to recognise borders drawn by the British as at time when China was weak and being carved up by Western powers.

Regarding Aksai Chin (Eastern Ladakh) even the British were not consistent, putting it in British India (Johnson Line) or China (Macartney-MacDonald Line).

Regarding Arunachal Pradesh the situation is quite clear from a Chinese point of view: This is a part of Tibet that was thus in China until the fall of the Qing Empire. At which point Tibet declared independence and the British took over Arunachal Pradesh from Tibet/China. Obviously this was never accepted by China.

These issues are never black and white.