> And what happens should they theoretically fail to pay? Does law enforcement show up at their door? Does a collection agency handle it? Court-assigned fines

> And what happens should they theoretically fail to pay? Does law enforcement show up at their door? Does a collection agency handle it? Court-assigned fines to the government generally get collected by the internal revenue service, not a collection agency; but the standard means if they do not comply does not involve law enforcement showing up at their door but the starting point is letters to all the banks to seize any current and incoming funds, if that is not sufficient, followed by seizure and auction of any real assets (e.g. buildings) - but since the described amount is roughly what Google earns from EU in a single month, simply seizing that sales revenue would be enough.