Does PressAndHold automatically combine region with general language?

The .plists in PressAndHold.app for en and en_US have different sets of <key< definitions with no overlap. Some have a different naming style. Do they get combined if a US locale is selected?

OR where is there any Apple documentation to explain the gory details of how this thing works? (searching developer.apple.com found nothing useful)

There are a lot of things here in Apple.SE and elsewhere that talk about specific edits, but I haven't found anything that explains some of the odd differences between the two files.

The biggest reason I'm interested is that all the pop-ups I can find a key for are specified as <key<Roman-Accent-[key]</key< but I'd like to find a way to get the pop-ups specified with UI.

   <key>UI-Currency-$</key>
   <dict>
      <key>Direction</key>
      <string>right</string>
      <key>Keycaps</key>
      <string>$ ¢</string>
      <key>Strings</key>
      <string>$ ¢</string>
   </dict>
   <key>UI-TopLevelDomain</key>
   <dict>
      <key>Direction</key>
      <string>left</string>
      <key>Keycaps</key>
      <string>.com .net .edu .org .us</string>
      <key>Strings</key>
      <string>.com .net .edu .org .us</string>
   </dict>

I can’t be sure, but I suspect the answer is NO.

I finally found some pages about how “firmlinks” can be created. One said I would have to specify them in /usr/share/firmlinks. Another said that file is only for directories from Apple and users must specify them in /etc/synthetic.conf.

Neither method worked, using the syntax in the pages’ examples.

But then I learned that the file I should modify is not actually in the read-only area. So I made the changes I wanted and tried it out. Didn’t work!

A little more investigation revealed that Apple put an identical file in the read-only area which overrides the one in the data area. Copied the one I had edited into the read-only area, and the pop-ups I had added worked.  Unfortunately, the next software update restored the factory default.