Learn Finnish: Pisarassa alkoholia on paljon energiaa. - In a drop of alcohol there is a lot of energy.

This is the core of the measure and container rule. When a noun acts as a measure or container (like pisara, a drop), the substance inside it (alkoholi) stays in the partitive case (alkoholia) no matter what case the container is in. So, while the drop gets the 'in' ending (pisarassa), the alcohol remains uninflected in the partitive.

Why does the sentence start with the location (pisarassa alkoholia) instead of the subject?

This is a classic existential sentence, which expresses that something exists somewhere. In Finnish, these almost always start with the location (pisarassa alkoholia), followed by the verb (on), and end with the thing that exists (paljon energiaa).

Could I use the genitive here and say alkoholin pisarassa?

You could, but it changes the nuance. The genitive (alkoholin pisarassa) sounds more like 'in alcohol's drop' (a specific drop belonging to the alcohol). When expressing a quantitative measure ('a drop of alcohol'), the partitive attribute (pisarassa alkoholia) is the natural and correct choice.

There are two words in the partitive here: alkoholia and energiaa. Are they in the partitive for the same reason?