Beliefs about the ethnic origins of non-immigrant ancestors are easy to come by.
They are more difficult to prove.
The best way to determine if a person has a certain heritage in their background is to research that person as completely as you can. Then do the same thing for their parents, grandparents, etc. Stories about where a person’s family came from are a dime a dozen. They are also difficult to prove the ethnicity is what you start with instead of the specific person.
I may think my relative was Native American and it may turn out that they are. But the best approach is to research that relative in as many records as I can find and see what those records say.
And go from there.
Asking help as needed and not being surprised if the information I locate is different from the story I’ve been told.