MS Azure App - Cross organisation (Multi Tenant) access to Sharepoint files

Question

Required: MS Graph App that has access to companyB.sharepoint.com drive/file items for a member of CompanyA

I am from Company/Organisation A. I have been granted access to a sharepoint site of Company/Orgtanisation B. https://companyB.sharepoint.com/sites/company_name/XXXX%20Files/Forms/AllItems.aspx

Via a browser this works as expected (i.e. access to the sharepoint site)

I have created a Multi-Tenant App (Azure dev portal) and have been able to receive a user delegated access token. refresh token works fine. The App (Overview) => Supported Account types: Multiple organisations.

Using the Graph Explorer (Authenticated for company A user) https://developer.microsoft.com/en-us/graph/graph-explorer

https://graph.microsoft.com/v1.0/sites/companyB.sharepoint.com:/sites returns

{
"error": {
    "code": "invalidRequest",
    "message": "Invalid hostname for this tenancy",
    "innerError": {
        "date": "2022-04-12T04:36:00",
        "request-id": "qqq21a6d3-xxx-xxx-xxx-xxx390a4yyy",
        "client-request-id": "qqq382fa-xxx-xxx-xxx-xxx708yyy"
    }
}

}

What I'm trying to achieve is to get access to and obtain a list of files contained in the Sharepoint site under CompanyB. Then being able to walk through the files, add/edit/delete files to these directories (or whatever terminology MS uses for Sites and Files/Folders within a site) I cannot seem to find the 'SiteID' or 'DriveId' that the URL references point to...

Would it be feasible to create an APP under Company B (ie create a dev account for them) and create a simpler, non verified, organisation only app under company B and then login as a user from Company A?

Any pointers of examples of anyone that has done this is appreciated.