How is the popularity/rank in functions.csv determined?

That's actually a value that Wolfram determines themselves. Here is the documentation for it: https://reference.wolfram.com/language/ref/WolframLanguageDa...

Here is e.g. all the values for the Plus[] function:

$ wolframscript -code 'WolframLanguageData["Plus", "Ranks"]' {All -> 6, StackExchange -> 8, TypicalNotebookInputs -> 5, TypicalProductionCode -> 6, WolframAlphaCodebase -> 6, WolframDemonstrations -> 4, WolframDocumentation -> 4}