Yeah, in the US fixed rate mortgages are handed off to the government who guarantees them and bundles them up into securities... that SVB then ended up buying a bunch of because it's not like there was exactly a wide variety of investment options available that they could back their deposits with that actually paid meaningful interest.
At least based on this article, it even sounds like this is what they wanted banks to do! The reverse-repo market was created because the Fed wanted banks to buy securities directly from them when interest rates where increased, sounds like SVB and others just followed that playbook
