From the documentation on How Geographical Data import works:
Analytics provides a number of geographical dimensions, such as City, Country, Continent, etc. The values for these dimensions are automatically derived from the IP address of the hit
See the link for full details.
Note: for this purpose there is no difference between "Google Analytics" and "Google Analytics for Firebase" as the location is determined in the same way, no matter how the event reaches Google Analytics' servers.
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