"In modern Supreme Court history only a few justices, such as former Justice Antonin Scalia, have regularly written their own first drafts.[17]"
All the other justices have significant input into their 1st drafts & subsequent drafts, including adding opinion to them.
You should read this! It tells just how much involvement the Justices have in shaping & competing their Decisions, including the writings that document them!!
https://en.m.wikipedia.org/wiki/Procedures_of_the_Supreme_Court_of_the_United_StatesEdited by STL1DFW at 00:50:10 on 02/01/17