At that point, there's an IB team handling the deal, an LLC holding the money, and probably several banks holding its accounts. Few banks are happy to handle that kind of concentration in a retail sight account.
Is it even possible to hold that much in a regular checking account?
yes, you can't stick with a 32 bit account any more, needs 64.
When I worked on banking systems for analyzing demand deposit accounts and ledgers we used Snowflake's NUMBER type
Money should never be stored using floating point variables. That is why the Decimal type exists and why IBM Mainframes have hardware acceleration for binary coded decimal math.
I'm guessing at those sizes the bank holding that much of your stuff becomes a risk for them so they have someone you can talk to directly to smooth over any potential problems