OP . . . if you have wire size limitation of 90A at the subpanel,
. . . . . . . just the HOUSE feeding the detached garage needs the 90A breaker.
No need to have 90A breaker (ie subpanel shutoff) as it can be 100A main breaker that may come in your 6 space, or 10 space, 12 space, 20 space, etc...