What's the difference between AAD Connect and Cloud Sync?
Cloud sync is great for merger and acquisition situations where some identities are required in the cloud but are not connected to the domain that hosts your primary DirSync instance (and you don't want to use AAD Guest Accounts).
Other useful points (updated);
SSPR for Cloud Sync is in preview (Q2 2022)
Trusts are illustrated to showcase the differences, they are not strictly a requirement for object syntonisation for a forest without a trust, AD port numbers and DNS can be used to reach these directories.
In future, Cloud Sync will become more powerful and have more features like Pass-Through Authentication and device object replication.
Complementary Material;
Azure AD Connect: Supported topologies - Microsoft Entra | Microsoft Docs
What is Azure AD Connect cloud sync? - Microsoft Entra | Microsoft Docs