I'm
guessing they all use Pacific Cycles manufactured and assembled frame+fork assemblies.
The frame decal claims only that they were designed in Germany (NTTAWWT).
Yes, when the Birdy was designed, Riese & Müller was a very small company with no manufacturing capabilities.
They made an agreement with Pacific Cycles that Pacific Cycles would manufacture the frame and that they could sell the Birdy on their own brand name in APAC.
Riese & Müller is still owner of the IP of the Birdy.
Each company makes its own choice of range of Birdy models and of the components mounted on every model of the range.
Nowadays, Riese & Müller has its own frame manufacturing capabilities in Portugal but the Birdy frame remain manufactured by Pacific Cycles.