First responders can see it, even when your phone is locked.
Your phone can show your medical details and emergency contacts on the lock screen, with no passcode needed. In an emergency, that's how responders learn your allergies, your medications, and who to call.
iPhone
Open the Health app and tap your photo.
Tap Medical ID, then Edit.
Turn on Show When Locked and add emergency contacts.
Android
Open Settings, then Safety & emergency.
Add medical information and emergency contacts.
Turn on lock screen access (the exact wording varies by phone).