Difference between kinds of items in Keychain, e.g., "Web form password", "Internet password", "application password", etc
Difference between different type of Password related to KeyChain 

App-Specific Password:

App-specific passwords are passwords for your Apple ID that let you sign in to your account and securely access the information you store in iCloud from a third-party app. For example, use app-specific passwords with mail, contacts, and calendar services not provided by Apple. App-specific passwords maintain a high level of security and help ensure your Apple ID password won’t be collected or stored by any third-party apps you use.

Web form Password:

These are passwords set by the site you are visiting, such as a bank for instance. They make the rules for their own site. Some do not allow your browser to remember for safety/security reasons. Others do allow it so you can set your browser to auto-fill those.

Internet Password:

These are passwords provided by you to access the Internet while connecting through WiFi, configuring Proxy and others that is related to network on your system. 

Keeping track of passwords is hard, especially if you follow the best practice of never using the same password twice and have multiple devices. iCloud Keychain keeps website and Wi-Fi passwords up to date across your Mac, iOS devices, and iPadOS devices. It also keeps account passwords and settings that you add to Internet Accounts preferences up to date on your Mac. 

How can I create a Keychain item of a specific kind?

Apps and servers may automatically create a keychain with a password for you when you create your account or log in, or they may offer you the option to have Keychain Access remember the password. It is the same for Internet passwords too as it is added automatically by creating a key chain under an appropriate group. 

You need to deep dive a bit on Keychain properties if you want to create for Web Form password for your own tool. It would provide a bit idea if you look on this query too.