A "population" within a journey is unrelated to the concept of populations within the Data Designer (the latter is sometimes also referred to as a "root data extension relationship").
As explained by SFMC within Data Designer:
Use a new population to segment a large group of contacts within your account. The Marketing Cloud recommends including only two to three populations within an account. Create a population only when you need to address a large and specific segment of your contacts.
Instead, a "population" within a journey simply refers to all the people who have entered that journey. This is the number displayed on top of your "flow" within the editor, and can also be toggled in the top right: