Whenever a university want to do a study on their entire students, thus the entire students are the meant population. That does not depend on what we are going to study: students' ages, disciplines trends, performance...etc
In industry, a company that produces pens of a specific type would like usually to re-evaluate the production. Here, their population is the entire production of pens which is obvious to be infinite. Their study may aim to examine major properties such as: length, shape, weight, life period...etc
OK, let's look for environmental example. The ministry of environment in some city used to check the pollutant concentration in a runny river. Based on the results they will advise people either to use or not the water of the river, eg for swimming, washing or irrigating. The whole water amount will be their population here that should be studied.