country flags are not showing in desktop

Hi,
The country switcher widget displays the flags using emojis. Unfortunately, a bug in Chromium and the derivate browsers (Chrome and Edge) on Windows affects the emojis flags :
https://issues.chromium.org/issues/40766658

This is why the widget option text says, “Display flags in supported devices.”

A workaround is to use the following code snippet to replace the select with a jQuery select2, that allows flags as images:
https://www.pricebasedcountry.com/docs/developer-docs/flags-currency-country-switchers-code-snippet/

where should I put the code snippet? In functions.php?

You can follow the WPBeginner.com tutorial to add the code snippet:
How to Easily Add Custom Code in WordPress (without Breaking Your Site)