Ahh. I knew about "estar" but I never knew that about "ser". Very good to know!

This little nugget would have come in handy when I was learning Spanish

And then locations and buildings come to fuck up that way of remembering it, because la biblioteca está allí, not la biblioteca es allí.

True that. I guess maybe it's because a building's location isn't necessarily part of its essence. Or how it feels more natural with respect to a person, e.g.: "yo estoy en la biblioteca" makes sense cause a person moves from place to place. I guess technically buildings can be moved but it's still a bit confusing.

The library is a thing, not a person. That's why you say está, because there's nothing inherent about a library, it's just an outer description. In general location descriptions are described with está, no ser.

Estoy triste: I am sad right now Soy triste: I am a sad person, in general.

La biblioteca está genial: the library is awesome right now (maybe due to an event or special decoration or because it was recently cleaned).
La biblioteca es genial: the library is awesome in general.

Tengo hambre: I am hungry
Soy hambre: I am hunger/(a hungry person)

Don't know, both would be ok, depeding on context.

• Dónde está la biblioteca?
• es aquí

Is totally fine, except if you are in a complex of buildings, or something like a mall or refering to the city (or similar) where the library is, then «está aquí» is the correct one.