Highly Impeded Magic Material Plane
This plane appears to be similar to the Material plane, but spells and spell-like abilities are quite difficult. Even at 9th level, the party is mostly failing to cast even cantrips.
Firearms appear to be commonplace, although swords and light armor have not disappeared.
Railroads also exist, and architecture allows the building of taller buildings than can be easily done on the default material plane.
Magic is practically unknown. Anyone using magical powers is likely to be called a witch and in danger for their life.
The party appeared near Piedra Lumbre. Their local guide (Jonathon Kinsley) told them of a distant land name Tsiang-Tzi. If the party claims to be from there, that will explain much of their weirdness.
Firearms are martial weapons for the residents of the plane. For others, the are exotic weapons.
Spells and spell like abilities require a check, the details of which the players do not currently know.
Supernatural and extraordinary powers work normally.