Modify the category text on the Add Devices page?

May I ask where these texts are modified?

Those are the class the device is under, they can’t be changed outside of the default device classes.

Can I create a new custom category then? Define the text of the category I want

Classes are predefined and not changeable, so no, you can’t create your own device classes.
If you are missing 1 that you think is really useful and/or used by a lot of devices you could request it to Athom to implement it.