Learn Russian: Каждый элемент представляет собой отдельный механизм. - Every element represents a separate mechanism.

Представлять собой is a highly formal way to say "is," "constitutes," or "represents." You will see it constantly in technical, academic, and official Russian texts. In everyday speech, you would just say Каждый элемент — это отдельный механизм, but the phrasing in this sentence sounds like an excerpt from a manual or textbook.

The lesson says представлять собой requires the Accusative case, but отдельный механизм looks like Nominative. Why is that?

It actually is in the Accusative case! Because механизм is an inanimate masculine noun, its Accusative form is identical to its Nominative form. The adjective отдельный matches this pattern. If the object were feminine, like система (system), you would clearly see the Accusative shift: представляет собой отдельную систему.

Does the word собой change if the subject is plural, like if it were "all elements"?

No, собой remains exactly the same regardless of the subject's gender, person, or number. It is the Instrumental case of the reflexive pronoun себя (oneself). If the subject were plural, only the verb would change: Все элементы представляют собой...

Could I drop собой and just say Каждый элемент представляет отдельный механизм?

Not without subtly changing the meaning. Without собой, the verb представлять usually means "to present," "to represent" (like an ambassador representing a country, or a lawyer representing a client), or "to imagine." Adding собой locks in the specific meaning of "to be" or "to constitute" something by its very nature.