On my iPhone I use Safari to log into my twitter account (I don't have the app on my phone). When I'm done I log out of twitter, so I'm very often using Safari's password saving ability.
I also use 1Password to save passwords (mostly these days because of its password generator). For work I had a set of passwords saved in a vault (1Password term) including multiple work twitter accounts. I've moved on to a different company and deleted that vault from 1Password. However Safari still lists those two twitter accounts in the list of possible logins to twitter when I go to the login page.
I deleted them from Keychain Access (on my Mac), and then searched on my phone (I'm guessing this was back when I was using iOS 11, because when I went to look for passwords under Settings > Safari just now it's not there anymore, it has its own list item in iOS 12) and did not find them.
iOS is definitely not still obtaining the passwords from 1Password. I've actually bought a new phone since deleting the work vault from 1Password.
How can I remove these usernames/passwords from my phone?
Related, but I'm willing to break this into a separate question if needed:
My bank website (on my Mac) also has an incorrect (old) username (no password) that autofills as soon as the page loads. I have to click the username field and select my correct username. This isn't as difficult because the UI is a little smoother, but it seems like there is some kind of login cache that I can't erase. I have removed this username from Keychain Access and it still shows up.
Best Answer
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