a) The commands to create the project do not mention neither meteor nor quasar
Every thing under api is managed by meteor. So for example to start the meteor backend you use Meteor CLI to start it. Also to update meteor you use meteor cli. And of course Quasar Cli is used to manage and start client.
The code in quasar-meteor/api/client/main.js/ contains Blaze code; why should it be needed it if the client is made of Quasar/Vue components?
I have not seen that, because I run meteor headless. But you could create a gui in the api backend part of meteor, for admin things. Its’ optional/just a bonus.
Is this even a valid approach and doable using the platform detectors, or it’s preferred to go with a separate project to serve the mobile app?
Yes it’s very doable with platform/mode detectors in a single project.
Here’s a repo/project/product(enFocus) that has a different ui/flow based on if you are on mobile or electron.
It is using different techniques to archive this( for example using functional components and different routes depending on what mode quasar is running).