Microsoft Cross-Tenant Sync vs. Multi-Tenant Org 365

We have a sister company, that we do not own nor controller their tenant. Managment would like to share some enterprise apps like SharePoint/teams/Intranet/ and power bi dashboards. I'm trying to figure out what the difference is between Microsoft Cross-Tenant Sync using Entra vs. Multi-Tenant Org 365 using the office 365 portal.

Since I don't trust their tenant how do we safely let them into our system to use those enterprise apps?

Has anyone used the Microsoft Cross-Tenant Sync through Entra? How does it work?

Has anyone used the Multi-Tenant Org through 365 that is still in preview? How is it working? Does it create the sync relationship on the Cross-Tenant Sync on the backend? Is there really any benefit from doing this instead of the Cross-Tenant sync only?