The resident population of an area is the people who usually live there. Where someone is on a given night does not matter. What counts is where they call home.
This is not the same as the enumerated population, which counts people where they are on Census night. Someone in a hotel far from home counts there, not back home.
The Australian Bureau of Statistics uses this idea to work out the estimated resident population. It takes the Census count, fixes it for undercount, and puts each person at their usual address.
Place Forecast uses resident population as its main measure. When the app says "population" it means the resident population. Estimated years come from the estimated resident population. Projected years extend this forward using the cohort-component method.