Convert matrix to 3-column table ('reverse pivot', 'unpivot', 'flatten', 'normalize')

Question

I need to convert the Excel matrix FIRST in the table LATER:

FIRST:

Answer 1

All of the solutions so far involve VBA, PowerQuery, etc. which are great, but are "one-time" events. To make it more dynamic, consider using INDEX(MATCH(...)). This will allow for dynamic updates to the table.