Under FDIC, any account less than $100K or so will be redeemed at par, even if the bank is completely and totally insolvent. Your checking account is actually probably backed 1:1 in reserves because your bank needs the cash on hand to match withdrawals at any time; it's your savings account that is less likely to be so reserved.

Furthermore, even in a fractional reserve banking scenario, your bank will have 1:1 assets backing it up, since the money it loans out is counted as an asset, although it needs to be recognized that the loan is valued at less than par because of the risk of default. There is quite a lot of legal regulations on what capital can back up accounts, and the minimum ratios of various kinds of quality of capital.

The FDIC limit is $250K: https://www.fdic.gov/deposit/deposits/faq.html

I believe the FDIC insurance was updated to 250k a decade ago.

Bank reserves are 1:1. If a bank's reserves ever dip below that ratio, the bank is insolvent, and is in deep shit.

Only a fraction of those reserves are cash. Hence fractional reserve. The remainder aren't liquid, but they are worth enough to cover the reserves, under best accepted accounting practices.

Tether's non-cash reserves largely consist of "A money launderer stole our money, but pinky swears that they'll give it back." It's not like a fractional reserve bank, it's just a straight up fraud. It's why despite many assurances from Tether, it has still not been independently audited.