就诊 is the formal/written way to say 'see a doctor.' In casual speech, people say 看病 (kànbìng) or 去医院 (qù yīyuàn). You'll see 就诊 on hospital signs and in official notices.
register
就诊 is formal/medical register. Casual equivalents: 看病, 看医生. You'll encounter 就诊 in hospitals, news reports, and health documents.