how to merge contacts from 2 iPhones

How can I merge contacts from two iPhones. I have a Xmax and a iPhone 12. I followed previous instructions and they were simple.

1. Have same Apple ID on both phones.
2. go to iCloud and turn toggle the "Contacts".
3. previous answers then say that I'll be promoted to "merge" contacts.

I was not prompted to merge contacts. Some contacts appeared on my new phone but not all of them.

More detailed instructions to merge contacts to my new phone would be appreciated.

Thank you.