"In the hospital" and "at the hospital" are phrases that are often used interchangeably, but they can convey slightly different meanings:
In the Hospital:
When someone says they are "in the hospital," it usually means they are physically inside the hospital building as a patient. This phrase emphasizes the person's presence within the hospital.
At the Hospital:
"At the hospital" can also mean being in the hospital as visitor. It can refer to someone who is visiting the hospital. This phrase is more about being in the general location of the hospital.