Learn Russian: Я люблю этот просторный зал, в котором есть камин. - I love this spacious hall in which there is a fireplace.

Который acts like an adjective that matches the noun it refers back to—in this case, зал (hall), which is masculine. Because the fireplace is located inside the hall, we need the preposition в (in). When following в to indicate a location, we must use the Prepositional case. The masculine Prepositional form of который is котором.

Since the verb любить requires the Accusative case, why don't the words этот просторный зал change their endings?

You are right that я люблю triggers the Accusative case for the object being loved. However, зал is a masculine inanimate noun (meaning it is not a living person or animal). For masculine inanimate nouns—and the adjectives pointing to them like этот and просторный—the Accusative form looks exactly identical to the Nominative (dictionary) form.

Why is the word есть included here? Couldn't we just say в котором камин?

You actually could say в котором камин, but adding есть puts emphasis on the existence or presence of the fireplace (literally, "in which there is a fireplace"). Without есть, it sounds more like you are just pointing out its location, whereas есть highlights the fireplace as a specific feature of the hall.