If asking about how to achieve the database schema, which you mentioned; then:
In Parse, it can be achieved in a similar way as in Firestore. But instead of having a subcollection of devices, you would have another table - Devices Table which will have its data. In the data, you would be having a field like userId
which will have the userId (corresponding to the user in Users Table). With the userId
in hand, you can query for the devices from the Devices Table.
If there are limitations on the number of devices per user in your app/website, you could possibly have an array of pointers (pointing to the device objects in Devices Table) in the original user object.