You can ask user to login to https://myapps.microsoft.com/<resource tenant name>
If user is part of tenant A and user is provisioned in tenant B as guest, then you can ask user to try URL https://myapps.microsoft.com/<tenant B onmicrosoft.com name>
This will directly take user to myapps portal of Tenant B and user will be able to see the collections created in Tenant B myapps portal.
Do let me know if you have any further questions.
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