Login keychain and Local items

What is the difference between the Login Keychain and the Local Items keychains? I have searched high and low but even Apple's own documentation or clear explanation is lacking on this!

I thought that keychain items are supposed to sync seamlessly across my iCloud connected devices but I don't see this happening reliably. For example I just saved login credentials for a website using Safari. I then go to my iPhone and go to the same website expecting the iPhone to auto-fill my password but nope. I checked under Settings > Safari > Passwords but the password I saved on my Mac isn't there. I am setup with a PIN for iCloud keychain.

Can anyone shed some light on this?!

Thanks.