Stucking on LOOKUP inside INDEX

thats just another way of building an array of values
you could put 1,2,3 in a1,a2,a3 =lookup(h2,a1:a3,{2,3,4})
then you could put 2,3,4 in b1:b3
=lookup(h2,a1:a3,b1:b3)
sometimes you see it in match
=match(h2,{100,101,102},0) if h2 had 101 in it it would return 2 the position in the array of the match