Otherwise I'd recommend reading from the source: https://www.fdic.gov/resources/deposit-insurance/ as there are lots of weirdness as one should expect from old complicated insurance systems ;)
But really, if you have more than $250k in cash, perhaps you should consider lowering your cash amounts. Certainly there are valid reasons to hold that much cash, but in general, most people probably shouldn't. If you put that cash in treasuries(even very short term treasuries so it's very cash like), you get paid interest for holding the bills, where most banks won't pay you much of anything for holding cash and treasuries are backed 100% by the US govt, unlike FDIC's 250k limit.