お役所の文書としてはここには結構きっちり書いてあるとおもうのだけど。

https://www.auswaertiges-amt.de/en/einreiseundaufenthalt/coronavirus

Exemptions from the requirement to quarantine:
Exemptions from the requirement to quarantine apply in particular to persons – except those travelling from areas of variant of concern – who:

have merely passed through a risk area or high incidence area (not an area of variant of concern) without a stopover,
are only passing through Germany and leaving the country via the fastest route available, or
in the case of border traffic: persons who have spent less than 24 hours in a risk area or are only entering Germany for a period of up to 24 hours.