I understand now.
If the control is not in the gallery then there's no way to add it to Control Center.
In the case of that specific toggle for Hearing Devices it does not appear to haver an associated control Center control that can be added.
You might be able to set up a Shortcut to get you there, and add it to the control Center.
Open the Shortcuts App, tap on the + icon to create a new shortcut, in the Search Actions search bar enter Open URLs and then tap it in the search results.
Tap on the light URL box and enter the following:
prefs:root=ACCESSIBILITY&path=HEARING_AID_TITLE (caps are important)
Tap on the blue checkmark to save. You may want to rename it to something easier to find by tapping at the very top and editing the name there. When you are done tap the < icon to exit the editor.
Then go to control center, and follow the steps to add a new control, search for Shortcuts, and tap on Shortcut, then on "Choose" and look for the shortcut you just created it should be there at the top.
Tap it to add it to the Control Center.
Then tap anywhere outside an icon to end the edit session. The next time you tap on the shortcut there it will take you straight to Hearing devices.
You may want to play around with the path to add the sub menus like "path=HEARING_AID_TITLE/AUDIO_ROUTING_TITLE/MEDIA_AUDIO_TITLE or similar; can't be sure what the actual path is since Apple does not document this and I don't own hearing aids to test the path with so I don't see the full path and can't get to it to see if it actually works. At worse it should at least shorten the amount of taps to get to the Media Audio controls.