Learn Russian: Во-первых, мы уточняем, чьи именно это отпечатки. - First of all, we are clarifying exactly whose fingerprints these are.

The particle именно adds the meaning of exactly or specifically. When used with question words like чьи (whose), где (where), or кто (who) in indirect questions, именно most naturally comes immediately after the question word, creating the set phrase чьи именно (exactly whose).

Could we use как раз instead of именно here to mean exactly?

No, как раз would sound unnatural here. While both can translate to exactly, как раз is used to point out a coincidence in time, size, or situation (such as arriving just in time or a shirt being a perfect fit). To clarify specific details, especially following a question word, you must use именно.

The word отпечатки is plural, so why does the sentence use the singular это instead of эти?

Here, это is acting as a standalone demonstrative pronoun meaning these are. In this pointing-out role, it is frozen in the neuter singular form regardless of the gender or number of the noun it introduces. You would only use the plural эти if it were an adjective directly modifying the noun, as in эти отпечатки (these fingerprints).

Why is there a comma after Во-первых?