Multi-Geo really refers to your ability to create content across multiple geographical locations. Not Necessarily anything to do with how the user accesses them.
3 years ago, if you were an international customer and you needed for compliance or legal reason to keep your European data somewhere in the EU, and your US data somewhere in the US, you were required to have two separate tenants, which essentially meant in Office 365 you more or less had two separate companies.
With Multi-Geo, you can have one tenant and choose to house certain data in certain locations. That does not however by itself limit who can access the data.
So at face value, your question is that they would access the data just like anyone else would. Say I have a US tenant, and someone from the UK wanted to access a SharePoint site. Well as long as they had permissions to the site, they would just navigate to the site as normal. You would just need to know that data housed there was housed in the US (in our example) so if you had any compliance needs keeping data in the UK you could not house it in sites on that tenant.
In order to provide restrictions to access (which I think your question is getting at), you would either need to just limit access for those users using traditional Office 365 permissions, or more holistically add in conditional access policies through either ADFS or AzureAD to limit access by location.
If Multi-geo was on, you could in essence choose where to create sites, and as such house data should you need to do so.