Keychain in iCloud, how does it work?

I have never activated Keychain in iCloud. I have four Macs, each with its Keychain saved locally. On the main Mac I have a lot of website passwords saved in Keychain, on the others fewer.

I have been looking for information on the best way to share passwords among all Macs and would like to ask if this is right:

- On the main Mac (MacPro 2013 - Monterey, the one with the most data saved in the Keychain) I enable Keychain in System Preferences > iCloud. Does this cause all those passwords to be saved in iCloud?

-I then do the same in the other Macs (Mac Mini M1 - Sonoma, MacBook Pro 2010 - High Sierra, MacPro 2009 - El Capitan). Does this cause all other Macs to have access to the same passwords?

The question is: what happens to the passwords that were saved locally in the other (older or newer) Macs? Will they be added in iCloud? Will I be asked which ones to keep if the same?