If I remember this right in the SS hierarchy Heydrich reported to Himmler. The actual “ architecture “ (planning) was done at the Wannsee Conference. Heydrich was there, I am not sure Himmler was.
The Wannsee Conference was called by the director of the Reich Main Security Office SS-Obergruppenführer Reinhard Heydrich to ensure all Ministries and departments co-operated in the Final Solution.
The conference was in response to a letter sent to Heydrich in July, 1941 by Vice-Chancellor of Germany Hermann Göring giving written authorization to Heydrich to prepare and submit a plan for a “total solution of the Jewish question” in territories under German control and to coordinate the participation of all involved government organisations.
As Vice-Chancellor Goring outranked Himmler.
Goring also held many political offices in the Reich which also outranked Himmler.
Himmler did use Heydrichs position in the SS to insure the Final Solution was run by him.
The Wermacht didn’t want it anyway.
Himmler may or may not have made a morning appearance at the first day of the conference.
Records of that are sketchy.
The letter from Goring to Heydrich, as well as the minutes of the conference, were used at Nuremburg and is on display at the Wannsee House which has been turned into a Holocaust Memorial.