In the movie Thor Ragnarok the sentence
Asgard is not a place, it's a people.
is used by multiple characters multiple times but shouldn't it be
" Asgard is not a place, it's the people"
Am I right or wrong?
Grammatically either would be valid, but not quite the same.
The people would mean a particular group of people, which in the context would mean "the people who live in Asgard".
A people would mean a particular group of people defined by some line of nationality, ethnicity etc. As in this definition:
(countable) Persons forming or belonging to a particular group, such as a nation, class, ethnic group, country, family, etc; folk; a community. Source
It can use the indefinite article because there are of course other "persons forming or belonging to a particular group".
The two are so close that it would have been reasonable to use either, but the choice of "a people" gives a better pairing to "a place" and so was probably the better choice.
"A people" is correct, because the purpose of this statement is to convey that the name "Asgard" refers not to the physical planet but to the race that inhabited it. The phrase "a people" can mean a race, ethnic group, or nationality bound by something so that they are sometimes referred to as a collective. "The people" implies that "Asgard" is the people of Asgard - anyone living on that rock, which again gives possession of the name to the location, not the Aesir, and also implies that "people" is used in the plural rather than the singular. They are one people. Also, the specificity of "place" and "people" need to agree for this statement to make sense. It says "a place," not "the place," so it must say "a people" as well in order for Thor and the others to accurately contrast the presumed meaning of "Asgard" with the actual meaning.
Because it meant the ethnic group or nation. It's a whole in one heart because they believe in Asgard. So, it's a plurality of persons considered as a whole.