Where does Safari store passwords? I am having a very severe issue. I erased my Mac’s disk and reinstalled macOS High Sierra yesterday (I was already on High Sierra before reinstalling). I made copies of the drive before wiping it and I have all the data of the old disk at hand on external drives. (This is not a question about any kind of data loss, no loss occurred.)

Just after my clean install I wanted to reinstall all the passwords I had saved in Safari (I just store them locally, without using iCloud sync). I was absolutely convinced they were stored in the session Keychain, so I just opened the clone of my old disk and then ~/Library/Keychains/login.keychain-db and I almost had a heart attack when I saw they were not there. There was a lot of stuff, including my WiFi passwords, iMessage, App Store, Steam passwords, certificates and so on, everything was stored there EXCEPT all the Safari Internet passwords. 
So I tried to copy the whole Keychains folder to my new install and replaced the new one with the old one. This had no impact. Still no passwords in Safari.
Because I don’t use sync services I have literally no way to access these passwords ever again if I don’t find the file where they were stored. Where does Safari 11 usually stores locally saved passwords on High Sierra? This is a very serious issue, I am locked out of literally everything including important documents.
Your login keychain should be where your passwords are stored. But it's worth noting that there is a "System" keychain in /Library/Keychains as well. Additionally did you log out/restart and log back in after replacing the keychains?