As for the backing, let me go through this in more detail (see https://www.bloomberg.com/opinion/articles/2021-06-10/maybe-... for a decent coverage of how this works). There are three categories to keep in mind here: assets (which include loans, cash, precious metals, etc.), liabilities (i.e., the value of all of your deposits), and capital (which is money to support the assets that's actually the bank's owner's).
Regulation requires liabilities to hit no more than about 90% of assets. They also require the bank to stake about 8% of its own capital to support the assets, on a risk-weighted basis. The risk weighting reflects the likelihood of assets crashing in value: a typical loan might have a 100% weight, so $100 in loans requires $8 in capital. Safer loans (e.g., residential mortgages) would require less capital. Stuff like cash or T-bills have a risk weighting of 0%. Riskier things include stocks (which tend to go down big when they go down), requiring maybe $24 per $100 of stocks, or cryptocurrencies... which require $100 of capital per $100 of cryptocurrency.
What this actually means, when you combine stuff together, is that you really do have something like $100 in safe things for every $100 in deposits a bank has.