> Are these mortgages backed by the underlying real estate?

Yes.

> 62 million / 850 = 80k per unit. Seems easy to cover?

I imagine if the owner could sell at that price they would. The value of apartment buildings, broadly speaking, is based on net operating income divided by a market-specific capitalization rate. The math might just not work for new owners based on the prevailing interest rate and rent environment, or the NOI and cap rate might make the selling price untenable for the current owners.