Translation:The girl from Oulu is watching a Canadian series.
(native speaker here)
The -lainen/läinen ending (lit. meaning "of kind") can be attached to pretty much every location name to show ethnicity or nationality (and used as an adjective). Sometimes the meaning has been shifted to a more special one.
- Oulu → oululainen
- Berliini → berliiniläinen
- Ruotsi → ruotsalainen
- Venäjä → venäläinen : there are some, very very few, speakers, who use the word venäjäläinen to denote citizenship of Russia and use venäläinen for the ethnic group (sometimes referred by isovenäläiset)
- Yhdysvallat → yhdysvaltalainen
- me → meikäläinen : one of us, of our people
The -lainen/läinen ending is a neutral expression, while tyttö Oulusta is emphasizing the origin, as if girls from Oulu wouldn't usually watch a Canadian series.
This is a tricky subject and totally out of the scope of the course, but it comes down to a question what is a state, a nation or an ethnic group.
Russia is a multi-ethnic state where live besides российский (ethnic Russians, isovenäläiset) a lot of other ethnic groups (tatars, tataarit, being the second biggest), so in the Russian language all are commonly called россия́не (Russian) which some, albeit very few, have translated to venäjäläinen.
When it comes to the USA, you might want to read an article "My Country Isn’t a Nation".
Of course I understand the concepts of ethnicity versus nationality. Even my thesis at the uni was about this topic but you put both in one bucket and the origin in the other and that brings confusion. What is the practical difference between -lainen and -sta forms in case of cities as in this sentence? Is the former about self-identification while the latter about the birth place?
Oululainen is in nominative and the oulusta is another form from which I do not know the name. That is translated as: from Oulu. In English you do not use the word Ouluner or Ouluist, so that is translated as from Oulu as well. Means the same, but just another way of saying it. At least that is what I understood
Oululainen refers to someone who or something which originates from the town of Oulu. So oululainen tyttö is a girl who has at least lived long in Oulu, if not born there, while in the expression tyttö Oulusta it is less clear what her connection is to Oulu. She might just have paid a visit there and now she appears in another location. All in all expressions of type tyttö Oulusta are rarely used and pretty much require a clarification about the connection.