When US officials term ISIS the "world's richest terrorist organization," they're not being hyperbolic. The group really is raking in huge amounts of money through its various sidelines, with Treasury officials saying they believe it amounts to tens of millions of dollars monthly.
ISIS has found myriad sources of revenue, from sales of oil wealth and ransom schemes to the age old practice of taxation, and that's not even counting the $400+ million they looted from the Iraqi central bank in Mosul.
Like most terrorists, ISIS started out dependent on the largesse of a handful of supporters, and the group continues to run a surprisingly successful campaign soliciting donations through social media. Large donations have been reported out of Saudi Arabia, Qatar, and Kuwait.
But as those nations try to close the spigot on the direct donations, it matters less and less for ISIS, which as a de facto nation state now has natural resources to exploit and millions of civilians to tax to pay for its assorted military adventures. As the US and others have shown in the past, that's a strategy that can keep wars going for quite some time.