I don’t think Betterment distributes funds to centralized accounts like Synapse does, it establishes accounts in your name at those partner banks and provides a unified interface over those funds. You can do the same thing yourself by going to each bank individually and opening up accounts manually, but then you’d have to deal with KYC for each one and their online banking interfaces of varying quality. That’s how they get their $2 million insurance number, since they open accounts at 8 banks and FDIC insures $250k per account.

That was my understanding when I looked into Betterment Cash Reserve (I ended up just going the manual route because I’m paranoid).

Good to know! Yeah I guess that's the inverse of the OP: one account spread to multiple bank accounts vs multiple accounts dumped into one bank account