In Germany the low-voltage grid is 0.4kV whereas the plugs in flats have 230V. Who is responsible for this conversion? Smart meter?
There is no conversion going on because it isn't needed.
The 400 V is the voltage between two phases of the 3-phase 380V grid.
The 230 V is the voltage between one phase and neutral.
Read more at: Three-phase electric power
LV grids in Germany and Europe usually consists on a three-phase connection with a voltage of 400 V (L-L). (Global exceptions)
However for harmonization residential distribution of the Low voltage grid is distributed to 230 V (L-N) for private households and continues using 400 V to others that need this voltage or 3 phase power.
Metering is a separate issue.
There are many different meter designs
- active power in 3-phase 4-wire network with service to 1 to 4
One to four tariff. Ability to measure power only in any two phases or in any one phase (when these options must be neutral connected).
- active power in one or two three-phase or four-wire network
One to four tariff. Ability to measure power in only one phase with the conditions attached neutral.