How do I download an accidentally deleted Passwords app?

According to Delete built-in Apple apps from your iPhone, iPad, or Apple Watch - Apple Support the Passwords app is not one you can delete. It is still on your device and you need to find it.

It can be hidden* by you, or by an administrator if your device is managed by an education or business institution. In the latter case you need to contact the administrator.

See if you hid it with restrictions:

Screen Time - Allowed apps Use parental controls on your child's iPhone and iPad - Apple Support

- "If you turn off an app or feature, it won’t delete it, it’s just temporarily hidden from your Home screen."

- Settings > Screen Time > Content & Privacy Restrictions > [Enter passcode] > Allowed Apps

- Settings > Screen Time, scroll down, then turn off "Share across Devices"

See if it is in your App Library -->

Organize the Home Screen and App Library on your iPhone [iOS14] - Organize the Home Screen and App Library on your iPhone - Apple Support

"Search for it ( Use Spotlight Search on your iPhone, iPad, or iPod touch - Apple Support ). Swipe down from the middle of any home screen and type the name of the app into the search field at the top of the screen. If it is in a folder, it will show the folder on the right of the app name after you tap on the app when it appears in the search."

"Reset the Home screen layout which will return the [app] to the home screen as long as it isn’t restricted. When you do this, it returns the home screen to out of the box condition and it will destroy any folders you had on the home screen. Settings > General > Reset > Reset Home screen layout."

* Use parental controls on your child's iPhone and iPad - Apple Support "If you turn off an app or feature, the app or feature won't be deleted, it's just temporarily hidden from your Home Screen."