How Does Mercury Work?

I don't use Mercury but I would have to assume your first sentence explains it as simply as possible

   Mercury is not a bank.

Any accounts they hold at partner banks are their accounts --- not your accounts. The only way your account at Mercury could be fully insured by the FDIC is if Mercury was a commercial bank --- and by their own statement and definition, they are not.

Therefore, any discussion of FDIC insurance is only relevant to them and their holding accounts. But since they are "not a bank", there is no requirement for them to use said insured accounts to hold your funds. They *could* be investing in crypto or Chinese stocks --- you really don't know. The only reasonable assumption is that how they choose to deploy your funds is entirely up to them and your deposits are not covered by the FDIC.

I'm sure someone will correct me if I am mistaken but unless there is an account at a commercial "partner" bank with your funds and your name listed as the account owner, you are not the FDIC nsured.