[1] you may want to search for terms like "regulatory capital" or "capital adequacy"
That's inaccurate. The entire idea of a bank is to connect supply of and demand for money across heterogenous creditors and debtors, which includes retail depositors and commercial lenders. There isn't a "secure savings account department" and a "risky commercial loans department"; it's all on the same balance sheet.
What does protect retail depositors (to a certain limit) are schemes like FDIC. Any retail deposits not covered by that, and any commercial deposits, will receive a haircut when the bank goes under. In the EU and UK, this has happened at least twice in the last few years.
Edit: Re-reading your comment I understand the confusion. I’m assuming Tether, if it were a bank, would function and be regulated like the Trust Department of a bank.
A bank has liabilities (customer deposits, financial contacts with other banks etc) and assets (loans, cash in a vault, their central bank balance, real estate of their office locations, office furniture etc). These are absolutely all on the same balance sheet, and the difference between the two is the bank's equity. Take a look at the Fed's balance sheet, for example: It explicitly lists the land that its branches are located on, and owned by it, as an asset [1].
What's regulated is, among other things, the required equity in relation to total assets/liabilities (capital adequacy requirements) and a high enough ratio of liquid reserves to total liabilities to handle various stress scenarios (liquidity requirements).
Importantly, and this cannot be stressed enough in this context, Tether is not a regulated bank.
[1] https://www.federalreserve.gov/monetarypolicy/bst_fedsbalanc...
The Fed is not a good example of how banks typically operate or how banking regulations are typically implemented because the Fed is not a typical bank.
12 USC 92a: https://www.law.cornell.edu/uscode/text/12/92a
12 CFR §9.13: https://www.law.cornell.edu/cfr/text/12/9.13
Securities are a different story – in that case, a bank or brokerage really only acts as a custodian for the accountholder, not adding them to their balance sheet.