Excel Month Sequences and auto population...the main cell information is a data validation dropdown

Hello, please bear with me as I try to explain this...
I am trying to copy the formula from a protected spreadsheet, and it has 2 main cells (darker shade of green) that can be edited (one for the year, manual text input) and (one for the month, a dropdown input).

When selecting the month, the below cells auto populate with the sequential months, in reverse.  So if December is selected, then November, October, September, etc... populates automatically.

Same thing when changing the cell for the year (2021). 

See below same selected month, just changing the year to 1997

Anyone can help figure out how I can make this same chart on my own?