[somerandomqaguy]

>My understanding is that in some parts of Canada it's common to plug your currently petrol/diesel car into an electricity socket overnight to provide a low-level of heating

Kind of. Block heaters help warm the engine coolant which warms engine and the oil, making it less viscous and help it flow properly so that the battery doesn't need to work as hard to crank the engine. They're typically only 400w to 600w draw.

Modern full synthetic oil and a well maintained AGM battery makes it less necessary, it's just a little less wear on the engine overall over the long term. Most folks though don't spring for either since they're a bit more expensive then mineral oil and flooded lead acid batteries.