use $ ti fix the row and column
=INDEX($AA$286:$AA$541,MATCH(B3,$Z$285:$Z$541,0))
its called relative and absolute addressing - if you wanted to lookup any training - websites - youtube sites to understand fully
the $ fixed that references
so
$A1
will not change the A - if fill down you get A2,A3,A4
But copying across you get A1,A1,A1
if you then fix the row only
A$1
will not change the 1 - if fill down you get A1,A1,A41
But copying across you get B1,C1,D1
fix both row and column $A$1
then no matter if copied down or across - will be A1 in all cells