We know that 100,000 immigrants are moving to the GTA every year. It would be enough for 10% of these to be singles looking for 1br condos in the city, to provide the demand for the construction. Now, it doesn't matter whether these are immigrants themselves, or people moving from the suburbs, being replaced by said immigrants. From anecdotal evidence, I know people whose children moved downtown from the suburbs, followed by themselves, all into condos, selling the house to fairly recent immigrants.
Anyhow, there are many mechanisms in play (people moving to T.O. from other parts of Canada as well), but it seems to me that the vast majority of people living in condos in Toronto are owners, and are usually young.
Not sure why this is a mystery to you. If Toronto is adding 10,000 units per year, that's room for approx 15,000 new people in the city per year, which can only capture a small portion of the region's population growth.
It is arguably a modest amount, frankly, since we are building way too much greenfield suburban type stuff to accomodate most of the growth (see Brampton).