For the duration, you sense the presence o f magic within 30 feet of
you. If you sense magic in this way, you can use your action to see a
faint aura around any visible creature or object in the area that
bears magic, and you learn its school of magic, if any. [...]
If this spell is used in presence of a magical item, does the aura assume the exact shape of the item, or just a general shape? The question can be read as: is the caster able to guess the item's shape by just seeing the aura?
"Around" means located or situated on every side: it does not contain within it any notion of how far from the object it extends or how tightly it conforms. Interpretation of this is therefore is entirely up to the DM. A tight, bright outline or a diffuse glow filling half the room both meet the definition.