The correct answer here is "all justices can write opinions that can be cited in future cases but only majority opinions are considered law."
Usually, the Chief Justice of the Supreme Court will pick one judge to write the majority opinion. However, each judge does have the ability to share their opinion. If a justice agrees with the majority decision and wants to write about a different reason why they agree, they can. If a justice disagrees with the majority decision they can explain why in a dissent. This really depends on the case at hand.
