They are different subjects. Anthropology is the study of human cultures and societies. Framing this as "ethnography vs. archival research" is a roundabout way of saying the same thing:
>Ethnography: The scientific description of peoples and cultures with their customs, habits, and mutual differences.
Ethnography means you embed yourself in a group, do a bunch of interviews, etc. It is a very hands-on and 1:1 kind of method. Your definition does not really adequately describe it. https://en.wikipedia.org/wiki/Ethnography#Features_of_ethnog... (Wikipedia isn’t a perfect source here, but gives some idea.)
This is a very different approach than sitting in an archive and reading old documents or running statistical analysis on government databases or ....
But it is possible to investigate the same types of subjects using these different methods.
>Ethnography: The scientific description of peoples and cultures with their customs, habits, and mutual differences.
https://www.lexico.com/en/definition/ethnography