It's hard to know exactly what you want here, but you don't really need "for each Object A, Object A Boo1 = 1" as only the triggered A objects are picked in the trigger anyway, So just picking B by VarA should work, or at worst, you need a For each A, then pick B by A.
Object A on trigger
-- for each Object A, Pick Object B by Object A. Variable
OR if you are adding to the list of "A"s by setting the Bool, then you need a Pick All A to get all of them before the For each that you have. Again, hard to know what you really want from the pseudo-code.