person admitted to a facility; inmate
Refers to someone who has entered and is staying in a care home, training institution, correctional facility, or similar establishment. The exact nuance depends on the type of facility.
The residents of that nursing home do exercises every morning.
The number of inmates in the prison has been decreasing year by year.
入院患者 specifically refers to a hospitalized patient, whereas 入所者 is broader and not limited to medical settings.
Compound of 入所 (entering a facility) + 者 (person).