Assalamu Alaykum ustadh, how do we interpret when Uthman رضي الله عنه made a second adhan for fajr. Were the companions allowed to add stuff to the religion of benefit because also the Hadith where the prophet ﷺ said bite onto the sunnah of himself and the rightly guided caliphs.

Ustadh AbdulAziz al-Haqqan

This has an asl the messenger did it for fajir. And as u said also the prophet affirmed the sunnah of the rightly guided khalifs.