Traveling from FRA (Frankfurt, Germany) to TPE (Taipei, Taiwan) via PEK (Beijing Capital, China) on a German passport.
Does the transit in PEK count as immigration thus requiring a visa or do TWOV policies apply?
You only need a visa or TWOV if you are not staying airside - which can happen in PEK Capital if you are transiting between T1/2 and T3. If you're staying airside, as a German citizen you don't need either.
And should you have to pass through Immigration to transit, you are eligible for a TWOV.
From my understanding, Germany has an agreement with China for visa-free transit stays up to 72 hours, so you should be fine stopping over in PEK without a visa as long as your connection time isn't too long. I'd double check the exact transit rules between those countries though.
Regarding transits that are not within airside:
German citizens don’t need a visa if their transit in PEK lasts for less than 144 hours. Source: The same applies to French citizens, US citizens as well as several other nationalities.
From https://english.beijing.gov.cn/travellinginbeijing/144hourvisafreetransit/mobile/: