(λx.x + 2) 1
→ 1 + 2
→ 3

id (id (λz. id z)) # the leftmost, outermost redex is the very first id
→ id (λz. id z)    # the leftmost, outermost redex is still the first id
→ λz. id z         # there is no outermost redex remaining, we're done.

id (id (λz. id z)) # the rightmost, outermost redex is the second id
→ id (λz. id z)    # the rightmost, outermost redex is the first id
                   # (it's textualy on the left, but there is no redex to its right)
→ λz. id z         # the only remaining redex is not outermost (it's contained within
                   # an abstraction) and cannot be reduced.

id (id (λz. id z)) # the leftmost, outermost redex is the very first id
→ id (λz. id z)    # the leftmost, outermost redex is still the first id
→ λz. id z         # there is no outermost redex remaining, we're done.