I have questions regarding license to integrate custom mobile apps with CDM.
If we customize our own mobile apps (iOS/Android, not Canvas App) to direct query/update data into CDM through web API, what would be the licensing like?
Can we just use a single user account for all external user, i.e. Customer to query/date data?
What you're referring to is multiplexing. Whether it's directly (via PoweApps) or indirectly (via custom app +SDK+integration account) accessing data, you need to license for internal users and external users appropriately.
More information is in the PowerApps licensing guide at the link below.
I did went through the licensing guide. However, it was not really mentioned licensing for external users with custom mobile app.
The scenario is something like this. Customer would like to have few portal for external users to access through browser, at the same time, a custom mobile app is needed as well so that it have the consistent data between the portal and the custom mobile app. In term of the portal licensing, it is required external user to register an account, so customer will be buying the 100 logins/ month subscription plan. But for custom mobile app, how should it be licensed? With the 100 logins/month subscription, does it cater for custom mobile app as well?
As long as the external user's are licensed (e.g. Power Portal login), then they should be fine to use that same login on your custom client (mobile, desktop, custom web).