Challenge 1: A Two-Dimensional Region/Quarter Lookup — Solution Walkthrough

Setup: A2:A5 = North, South, East, West (rows 2-5). B1:E1 = Q1, Q2,
Q3, Q4. B2:E5 = the sales grid, so row 3 (South's row) intersects
column D (Q3's column) at cell D3.

Formula:
  =XLOOKUP(G1, A2:A5, XLOOKUP(G2, B1:E1, B2:E5))

With G1 = "South" and G2 = "Q3", this returns whatever value is in D3.

Trace, inside out:

Step 1 — the INNER XLOOKUP resolves first, since it's an argument
the outer XLOOKUP needs before it can do anything:
  XLOOKUP(G2, B1:E1, B2:E5)
  = XLOOKUP("Q3", B1:E1, B2:E5)
  "Q3" is found in the 3rd position of B1:E1 (B=Q1, C=Q2, D=Q3).
  This returns the ENTIRE 3rd column of B2:E5 — that's column D,
  i.e. the range D2:D5 (North/South/East/West's Q3 figures).

Step 2 — the OUTER XLOOKUP now has a real range to search, not
another formula:
  XLOOKUP(G1, A2:A5, D2:D5)
  = XLOOKUP("South", A2:A5, D2:D5)
  "South" is found in the 2nd position of A2:A5 (row 3 overall).
  This returns the 2nd value of D2:D5, which is D3 — South's Q3
  figure specifically.

Explanation:

This is exactly the "resolve the innermost function first" reading
strategy from Chapter 1: the inner XLOOKUP's job is entirely about
narrowing the FULL grid down to just one column (Q3's column) before
the outer XLOOKUP ever runs. Once that's done, the outer XLOOKUP's
job is simple — find the right ROW within that single already-
narrowed column. Neither XLOOKUP ever has to search the full
two-dimensional grid at once; each one only ever searches a single
row or a single column.