The US was a safe place with consistent returns for decades. Now the US has increased risk. It isn't a calculation of "where do we put $100B", it's a calculation of "what is the balance of managing risk and returns".

Canada and Australia are low risk, but don't have the returns of the US. Brasil, Argentina, have higher risk.

The allocations get spread across different risk profiles, and the money gets spread across different investments. Of course, this is on the assumption the majority of this money stays in Treasuries. These large funds have options beyond that. It's a mathematical calculation where every asset in the world is an option. $100B can be spread around pretty easily I'd think.