It depends on where you live, here in the Netherlands these days some plastic flexible tubing is used, but about 15-20 years ago this wasn’t allowed and it was copper tubing everywhere. A little longer ago it was also soldered tubing only, they changed that because a lot of gas lines run below floors in older houses and soldering with a burner is dangerous there. Anyway, if you have a house which was built before 2000, you probably have copper gas lines over here.
It depends on where you live, here in the Netherlands these days some plastic flexible tubing is used, but about 15-20 years ago this wasn’t allowed and it was copper tubing everywhere. A little longer ago it was also soldered tubing only, they changed that because a lot of gas lines run below floors in older houses and soldering with a burner is dangerous there. Anyway, if you have a house which was built before 2000, you probably have copper gas lines over here.