How does Marketing Cloud Connect's Synchronized Data Source work for Leads and Contacts?

Marketing Cloud Connect's Synchronized Data Sources create read-only DEs in Marketing Cloud that mirror CRM object data:
Supported objects for sync: Contact, Lead, User, Account, Campaign, Campaign Member, and configurable custom objects.
Sync frequency: Contact and Lead records sync near-real-time (within minutes of CRM updates) via streaming events. Other objects may sync on a scheduled basis.
Configuration: In Marketing Cloud Setup, select which CRM fields to sync for each object. Fields selected appear as columns in the corresponding synchronized DE (e.g., Contact Salesforce Data).
Usage in Journey Builder: The Salesforce Data entry source can use synchronized DEs as the entry audience. Records matching filter criteria on the CRM-synced DE enter the journey automatically.
Limitations: Synchronized DEs are read-only — you cannot write back to CRM via the DE. Use the Sales/Service Cloud Activity or a Flow-based integration to update CRM records. Large orgs with millions of records should carefully plan initial sync time and the ongoing delta sync window.