Apple Keychain Features

Yes. You can manually add passwords to Keychain Access by clicking on the note and pencil icon at the top of the window.

No, Keychain Access items are not viewable on an iPhone or iPad. Only on Mac.

Store confidential information securely using Keychain Access on Mac - Apple Support

Because "Keychain Access" is a Mac only feature.

If you want interconnectivity for passwords with iPhone / iPad then use the Passwords option in System Preferences->Passwords.

If iCloud Keychain is turned on in System Preferences->Apple ID, then passwords added there will sync to all devices over iCloud.

Link-> Set up iCloud Keychain - Apple Support

It's a bit confusing because they are labeled similarly, but are not the same thing.

You can use the Notes app on both MacOS and iPadOS and iOS platforms and use iCloud for notes to sync them. Put a password on the note to secure (just DO NOT forget it a Notes password cannot be reset or recovered for existing notes).

Or you may want to look into a third party solution like mSecure (which I use and have for years now - syncs across MacOS, iPadOS, iOS, & Windows) or 1Password or similar corporate third party software that has a lengthy history of stability and support (avoid cheap apps especially those from single individual developers as they tend to get dropped without warning leaving your information unavailable).