I was reading about this and most banks apparently do some amount of diversification with the customer deposits, whether it be through loans, mortgages, whatever debt. They're only required to have, say, 10% on hand/immediately available to the customers. This supposedly has worked almost all the time, unless everyone wants their money back immediately.

SVB did riskier things with the customer deposits, they got long term bonds (kinda strange for a bank to go all in on that) and the Fed raised interest rates which they haven't done for a long time. People suddenly wanted their money and spoke into existence this bank run, and SVB's current value of those bonds was less than they paid for them, which means they suddenly went "oh, we don't exactly have all of that money at the moment, and we won't."