Google-sheets – How to reference a cell from a Google Spreadsheet in a Google Document

That's not possible at this time.

Instead use add the IMPORTRANGE formula in a sheet and use the local reference in the Data Validation dialog.

If you don't want that the imported range be at simple sight, you could add a sheet to be used for the imported range and hide that sheet.

Spreadsheet functions cannot access the content of other spreadsheets, other than by invoking importrange.

But you can have importrange inside other functions. For example,

=join(",", importrange("...", "Sheet1!A1:A5"))

returns the content of cells A1:A5 in the other spreadsheet, comma-separated within one cell. Another command often nested is query, which allows you to do a lot of processing of another spreadsheet's data without getting intermediate results in. For example:

=query(importrange("...",  "Sheet1!A1:B9"), "select Col1, sum(Col2) group by Col1 order by Col1") 

returns the totals of the second column split by the values of the first column, and sorts by the first column.