Power Apps (Both Model-Driven and Canvas) can be shared across Office 365 tenants using Azure B2B. But there is no way to choose a tenant in Power Apps mobile app in order to see/access the Power Apps shared with users of other tenants as guest users.
For instance, I have a case where the model-driven app has been shared with me from the customer O365 tenant using Azure B2B and I can access the app perfectly fine from the web browser (Single Sign-On). But I cannot access the same app using Power Apps mobile app. There is no provision in the mobile app to choose the tenant or to access cross tenant shared apps.
I see this as a major limitation where there is no use of Power Apps mobile app. The user can access the browser to access the app but he will not get the features of the Mobile app like Offline access, mobile controls (camera, etc).
It would be good if we have a selector in Power Apps mobile app where after choosing the tenant we can see the apps shared with me.