Japan had the Treaty of Shimonoseki to claim on Taiwan in 1895. Did China (ROC or PRC) has any effective LEGAL DOCUMENT to claim on Taiwan after WW2? NO! That's WHY Taiwan is not a part of China. That's WHY today's PRC is representing China without Taiwan! Beijing's claim on Taiwan is 100% bogus. China's claim on Taiwan is null and void! Chinese people, don't get cheated by the CCP any more!
This is the most important understanding about Taiwan: Taiwan is Taiwan. Taiwan is not a part of China (ROC or PRC). Taiwan is Taiwan. The "ROC" is but a piece of cloud above Taiwan! Do you remember that song saying "I am a piece of cloud in the sky ...... " Good luck!