Keychain and another Mac

Hi. I am a bit confused about the use of Keychain.

When creating new web accounts (newspaper etc) I use the Strong Password suggestion by Safari. I assumed this was kept in my Keychain.

This week my Mac packed up and had to be sent in for repair.

I have borrowed a new MacBook but now I cannot log in to the web pages due to not knowing each password. I wish I had simply used some memorable - but less secure - passwords.

So I am thinking what was the point of using the Safari suggested passwords and why isn't my Keychain data in iCloud (or is it?)?

Also, Secure Notes in Keychain seem reside on my broken Mac. Maybe I will I have to access the files via its back up HDD ... but again I thought this was all saved in iCloud. My mistake?

Thank you for any clarification.