When does a visa extension in Thailand take effect: is it from the old visa's expiration date or the new application date?

This is how it USUALLY works people. IF you are applying for an extension for the same reason as the one you're on the new extension will be added to the expiration date of the stamp you have now.

BUT

IF you are applying for an extension for a DIFFERENT reason <- like going from a visa exempt or tourist visa stamp/extension to a covid extension USUALLY they start the new extension on the date you apply because it's for a different reason <- meaning you would lose any days you have left on the current stamp/extension if you switch reasons and go apply early

as a rule extensions are added to the date your current stamp expires UNLESS you're changing reasons for your extension and then they start on the day you apply.