@Anonymous Thank you for reaching out to us, As I understand you are looking for details/approach in expanding the organization from US to Europe with Identity Access Management.
Researched on this requirement, came across this option
The Microsoft 365 Multi-Geo Capabilities add-on provides Enterprise Agreement customers with the ability to expand their Microsoft 365 presence to multiple geographic regions within a single existing Microsoft 365 Tenant. Multi-Geo enables customers to manage data-at-rest locations at a granular level for their users, SharePoint sites, Microsoft 365 Groups, and Microsoft Teams teams level. Multi-Geo is targeted to customers who have a need to store data in multiple geographies to satisfy their data residency requirements.
https://learn.microsoft.com/en-us/answers/questions/589408/for-gdpr-how-does-multi-geo-work
https://www.youtube.com/watch?v=3d9-Vt2fArk&t=291s
If this approach doesnt meet your business requirements, let me know, will check with my team further on this.