Learn Spanish (Latin America): Voluntarios, hay mucho plástico; separen las bolsas ahora. - Volunteers, there is a lot of plastic; separate the bags now.

Because it is a command! To tell a group of people (ustedes) to do something, regular -ar verbs like separar swap their usual ending for -en. If you used separan, you would just be stating a fact: "you all separate."

Is separen a formal or informal command?

In Latin America, it is both! The ustedes command form (separen) is used anytime you are giving an instruction to a group of two or more people, regardless of whether they are close friends, volunteers, or complete strangers.

Why does it say mucho plástico instead of muchos plásticos?

In this sentence, plástico is used as an uncountable material (like "water" or "sand") rather than counting individual plastic items. Since it is treated as a singular, masculine concept here, it takes the singular mucho (a lot of / much).

A quick refresher: why do we use hay here instead of está or es?

Hay is a special, frozen word that means "there is" or "there are" to simply point out that something exists. You use hay to state that "there is a lot of plastic" existing in the area, whereas es or está would be used if you were describing characteristics or the location of specific plastic.