Sync iMac and MacBook Pro?

Here you go when it comes to Photos and Desktop and Documents and Other Important Items: https://support.apple.com/guide/icloud/what-is-icloud-mm74e822f6de/icloud

What does "sync" mean to you exactly?

Macs do not actually "sync" to each other. They sync to iOS or iPadOS devices.

But you may share some data via iCloud.