Safari Password Cache?

There are a few possibilities of where the passwords are being saved. To clarify, do you know if you are using iCloud Keychain on your devices? Keychain Access works in collaboration with iCloud Keychain, which allows you to share keychains with your other devices. So it's possible they are syncing between devices via your Apple ID if you are signed in and have enabled that feature.

You can check the saved passwords on iOS as mentioned in ankiiiiiii's post. By going to settings, Accounts & Passwords, the looking under App & Website Passwords.

On your Mac with OS X Mavericks 10.9 or later with Safari open, Choose Safari, Preferences, then click on Passwords. You will have to input your computer password, then you can select a website to view your saved password for it.

Here are Apple's resources about this topic:

Set up iCloud Keychain

What is Keychain Access on Mac?

Get help with iCloud Keychain