> All I know is that you'll need higher amperage with lower voltage, but it can compensate proportionally.

This discussion is assuming that the amperage is the same. The common "default" AC socket in the USA is AFAIK the NEMA 5-15, which is a 15A socket; the common "default" AC socket in other countries is AFAIK usually either 16A, 15A, or 10A, so at most you'd have one extra ampere.

Of course, if you're adding a dedicated socket where you'd expect to plug a car (or other high-power devices like a large air conditioner), you'd put a higher-power socket like a 20A one. But this discussion is, as far as I understand, about what you could find on a random garage; I believe it would be unexpected to not have at least one common "default" AC socket on am enclosed garage, but finding a higher-power AC socket would be less likely.

Any idea how common or feasible it would be in the US to get ~400v 3 phase power to the garage?

Here in Australia it's possible, though not very common. ~400v is useful for some larger sized machinery.

At the same amperage, it's exactly twice as good, but if you're going to get a 220V outlet installed, you're also going to get a higher amperage circuit installed at the same time, so you can charge at 220V/50A instead of 110V/12A, which is 8x faster.

But you need also the electric company to be able to deliver that power, roughly 10kW, in addition to all your other electric appliances.

Common contracts - at least here (Italy) - are for 3 or 4.5kW, a few are 6 kW, 10 kW or more are rare, and you probably need to get to 15kW to be able to have those 10kW available for charging.

Anything less than 48kw is essentially unheard of in new construction in the US, and even old houses with 24kw service are getting upgraded. Less than 24 is considered essentially unsellable.

In new constructions, but what about the existing neighbourhoods?

Same thing. If your house was built after WW2 it will usually have 200a service if you're all electric, and maybe 150 if your heat is non-electric. Most older houses have also been upgraded at this point too. It's not uncommon for larger houses to have 400a (96kw) service.

It’s more than twice as losses are lower boosting 220v to the 400v or 800v the pack actually needs.

As an example, a charger in some EVs is 87% efficient when charging from 120v and the same charger is 94% efficient when charging from 240v.

The battery heater is a fixed overhead, so even at the same amperage (and twice the power), if the weather conditions are such that the heater was taking 80% of the available power on a 15A/110V circuit, it'll only be taking 40% of the available power on a 15A/220V circuit, meaning the charging power is 6 times greater.

I don’t know about US but here in Northern Europe the utility outlets we use for stoves and such are not just higher voltage but also distributed over 3 phases rather than just one. This translates into sqrt(3)=1.7 times more of the cable can be used. For a 3x16A outlet you get 400V x 16A x 1.7 = 10kW.

Compared to a normal 220V 10A outlet where your max is 2kW.

Higher voltage also means you get more power for less current, less heat loss in the cables.

If you have an electric car, the example above should explain why you want to get a proper charger with proper wiring installed. Don’t use the ones which plug straight into a shuko.