Nationalities - When do we use the singular or plural form

I always have doubts whether to use a singular or a plural noun when I refer to certain peoples. For example, we say Americans, Italians, Brazilians, Russians and Austrians. But we say The British, The English, The Portuguese, The French, The Spanish, The Chinese, and The Irish. When it comes to those nationals I rarely hear of, I'm never sure. These would include people from countries such as Azerbaijan, Armenia or Kenya.
Is there any rule as to whether we should use singular or plural ?

Sometimes the demonym matches the adjective, like with American, and then you say "The Americans".

Sometimes it doesn't as with Briton, and then you don't. This is particularly common with demonyms that ended in -man, such as Irishman, Frenchman, etc. (often avoided in gender-neutral contexts today).

Unfortunately, there isn't a way to know which is which without learning the demonym in question.

Fortunately, those that don't fit the productive plural pattern are mostly those that are European and have a longer history of use in English. You can be safer with places further afield or more recently founded:

Azerbaijanis, Armenians and Kenyans all fit the plural form.

Note too, that this form allows the singular without offence ("an American") while with those that use a collective noun ("the English") while some dialects allow the singular ("an English", "an Irish", "a Dutch") to some other dialects this is incorrect and can even come across as an insult.