A lot of this info is only partly correct and up to date.
In the USA, the Borg no longer charges interest on its loans. That part of the money-making scheme has been transformed. Because the congregations do not pay interest, their loans are often paid off fairly quickly. Then what? Then many times the congregation will pass a resolution to KEEP PAYING the Borg some sort of regular monthly "donation" since there is no good reason for the rank and file to stop donating and start saving money for their own family. One congregation I was in (and these numbers are guesstimates from memory) paid off its loan, which had been something like $850 a month, then agreed to keep paying the society $500 a month as a donation to the work and sold it to the congregation as doing the right thing and saving $350 a month off what they used to pay.
Also, the part on the Watchtower holding the deed is a bit confusing maybe, but it really is pretty simple. The congregation owns the Kingdom Hall as long as they exist as a JW congregation wanting to use it as a Kingdom Hall. If the congregation dissolves, then the Watchtower Society owns the Kingdom Hall and all other congregation assets. So how does this benefit the WTS? Example.
A decent-sized town has, for example, five congregations in it with two Kingdom Halls (call the congs North, South, East, West, and Spanish). Thanks to JWN, jwfacts.com, and Combating Cult Mind Control there are fewer than 20 elders and 12 MS left to handle all those congregations. Also, neither of the Kingdom Halls are very new and one of them was built back in the 1980s.
So the CO decides the area needs a Borg revitalization project. He forms a plan. Find a relatively central location and build a double-hall on it. Take the four English congregations and merge them into three, and split the Spanish into two congs.
What does this accomplish? Well, if in this restructuring the congregation that owned one of the previous KHs dissolves, what happens to their property? That's right, the Watchtower owns it, sells it, and pockets the money, often loaning it back to the congregation(s) for the building of their new KH.
And the cycle continues...