the fine has been calculated on the basis of the value of Google's revenue from search advertising services on Android devices in the EEA.
In other words, Google had an unfair advantage for search on android devices and leveraged that into revenue. The fine is a percentage of that ill-gotten revenue.
There’s an official guideline for calculating damages when anticompetitive behavior is found: https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:52...