90Tiger helpfully provided a link in this post:
http://tigerboard.com/boards/view.php?message=19230191
which explains how the SSP program is funded.
From page 6 of the linked document:
“ In response to the flows of migrants at the Southwest Border, on December 29, 2022, Congress enacted a full-year appropriation for DHS and FEMA. The Act directed CBP to transfer to FEMA $800 million from its FY 2023 Operations and Support appropriation, “to support sheltering and related activities provided by non-Federal entities, including facility improvements and construction, in support of relieving overcrowding in short-term holding facilities of CBP.” The Joint Explanatory Statement accompanying the Act explained the $800 million is appropriated for a “Shelter and Services Program” for FEMA administration. “