Learn Russian: Весь этот десерт содержит ровно шестьсот калорий. - This entire dessert contains exactly six hundred calories.

It is in the accusative case because it is the direct object of содержит (contains). However, for numbers counting inanimate objects, the accusative form is exactly the same as the nominative. You will only see both halves of the word change when you use other oblique cases, like the genitive (шестисот) or dative (шестистам).

How is the word "шестьсот" formed, and why does it end in "-сот" instead of "-сто"?

Hundreds from 500 to 900 are built by taking the base number (here, шесть) and adding сот, which is the genitive plural of сто (one hundred). It literally means "six of hundreds." By contrast, 300 and 400 use the genitive singular (-ста), which is why we say триста and четыреста.

Why is the word for calories ("калорий") in the genitive plural?

Калорий is the genitive plural of the feminine noun калория. As a refresher: when a number in the nominative or accusative case ends in 5 or above—including round hundreds like шестьсот—the noun that follows it must be in the genitive plural.

Could I use "целый" instead of "весь" to say "this entire dessert"?