How to display the right device name on Strava?

The expected location for the manufacturer id and product id in a FIT file is the file_id message. Strava's documentation shows that they read the manufacturer and product ids from the file_id message, and this is true for other platforms as well.
https://developers.strava.com/docs/uploads/

The example code for creating FIT files shows how to set the values in the file_id message.
https://developer.garmin.com/fit/cookbook/encoding-activity-files/

There is addition information on the file_id message in the the File Types section here.
https://developer.garmin.com/fit/file-types/

The ids are integers and including them in the file_id message does not mean that the platform consuming the file will have a mapping from the numeric id to a display name. For that you need to work with each platform directly.

Strava provides instructions on how to have your devices added to their database. Look under the "Device Mappings" section on this page for more information.

https://developers.strava.com/docs/uploads/