In my opinion, both have their merits. The manga offers more depth and detail in the story, but the anime brings the characters to life with dynamic visuals and sounds. It's hard to say which is strictly better.
The Horimiya anime didn't fully adapt the manga. There were certain plotlines and details from the manga that weren't included in the anime. However, what was presented still provided an enjoyable viewing experience for fans.
The Horimiya anime wraps up corresponding to a certain chapter of the manga. Unfortunately, I don't have the exact chapter number right now. Usually, anime adaptations might make some tweaks or omissions compared to the original manga.
In general, the Horimiya anime is quite true to the manga. It successfully captures the key elements and flow of the story. There are some extra scenes and slight tweaks for better pacing and animation, but they're not significant and don't go against the manga's essence.
The Horimiya anime doesn't cover the whole manga. Some storylines and details were left out for various reasons, like time constraints or creative choices.