The key point is that these can typically be readily converted into cash the company can use. So, while Apple has roughly $30 billion in actual cash and equivalents, this figure swells to more than $162 billion when considering marketable securities.
Second, it's important to realize that aside from cash and marketable securities, other values listed in the assets section aren't set in stone. For example, there's no guarantee Apple could sell its property, plant, and equipment holdings for the $43.7 billion listed.
The $6.33 billion in inventory listed assumes it will all sell for full price, and the $29.5 billion in accounts receivable assumes 100% of Apple's customers will pay their bills. And "other assets" is the vaguest of all, typically including the value of things such as patents, goodwill, and other difficult-to-value items. With that in mind, we can see that Apple has a total of about $352.6 billion in assets on its balance sheet.