I don't understand, why have some of the newcomers become builders and doctors?
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There are multiple reasons.
(1) Canada over-indexed on low skill immigration. High skill white collar professionals either directly immigrate to the US, immigrate to Canada with the intent to move to the US eventually or they stay in their own countries because COL in Canada's major economic centres is astronomical.
(2) Supply of doctors is artificially constrained by the governing bodies. It is much more difficult to become a physician in Canada vs the US. Salaries are also substantially lower. No surprise that the ratio of available doctors to patients is concerning.
The housing crisis in Canada is a complicated topic and I'm not informed enough to offer a summary of what led us to this mess. But this Wikipedia article [1] provides a good overview.