Isnt it that nowadays usually on the side of the driving seat? Or does this apply only to EU vehicles?

That would mean designing two separate entire fuel tank placements, fuel lines, etc for cars that are available both in left- and right-hand drive variants, with different SKUs for each of the parts needed. There is no way a car manufacturer would do that.

Im not aware of such a convention, I'm in the EU and most cars I've owned or driven has it on the opposite side of the driving seat.

Might just be a coincidence

Usually, it will be where the passenger side is in the cars home market. That is left for Japanese and British vehicles and right for US and German ones.

Fun fact, for single exhaust cars, the exhaust will usually be on the driver side, in order to route around the fuel tank :-)

I think it depends. Especially with PHEVs, which also have a charge port, whose location is determined by charging infrastructure, and which is not IME on the same side as the gas tank opening.