You grossly misunderstood what the single market means - there are of course absolutely different local regulations and customs that are used. Ireland (and the UK before they left) drive on the other side of the road, trains systems vary by nation, even electrical standards do - the single market in terms of norms means that they just have to all follow a general market admission will follow the same rules - e.g. a product needs to fully comply with the basic marke wide ruleset.