Here in the US, the barometer setting used is either the local setting when below 18,000'
MSL or 29.92 when at or above 18,000' MSL. This is according to FAR 91.81.

My question is on understanding how this works in Europe (and other airspaces
besides the US). My current hazy knowledge is that the 18000 feet may be different
either by country or some other boundary. Could someone familiar with this crossover
in Europe or elsewhere advise me on how it works?

Thanks in Advance!

is that in Europe