Eli5: What determines if a nationality ends with -ese (chinese, japanese…), -an (Ghanian, Egyptian, American) or just the county itself (Thai)?

Question

Eli5: What determines if a nationality ends with -ese (chinese, japanese…), -an (Ghanian, Egyptian, American) or just the county itself (Thai)?

In: 4879

Answer 1

On a similar note, why are people from Canada called “Canadian” with an “i” but people from Sparta are called Spartans with no “i?” Where does the “i” come from? People from Nevada are Nevadans afaik, not Nevadians.