What are the steps that a spreadsheet uses to communicate with other spreadsheets?

Spreadsheets communicate with each other:

1) With formulaic links to specific workbooks, specific sheets, and specific cells.

Links can simply be directly reflecting a value from one cell in one workbook into the current cell, or many Excel functions can be applied to the values of cells in other workbooks to bring completely different results into the current workbook.

2) With VBA programming

Macros can be used to open other workbooks, collect or create data, and place the data into the current workbook (or multiple workbooks) without the need for realtime formulas.