1. They come with some money, enough for whatever minimum down payment is required (but notably, the issuing bank will also lower down payment requirements for clients with high income or assets, which are faked anyway)
2. HSBC is incentivized to issue mortgages, yes, that's their business. But the actual fraud here sounds more like cash kickbacks from the fraud buyer to the issuing agent themselves.
3. Home prices are made on the marginal sale, so a small amount of this activity can have a large upward pressure on prices. This leads to typical bubble scenarios, and you can keep rolling over or refinancing mortgages as prices keep rising (or even just sell). To get a sense of this, over the past few years, the average Canadian house gained in price something like double the average Canadian income.
So in summary, a lot of this could be explain by plain fraud, enabling foreign buyers to both perpetuate, and participate, in a giant housing bubble. If that's true, and it all comes crashing down, then god help us all.