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)
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)