Your mileage may vary depending on the browser version (MS Edge vs IE etc.) so it would be best to do some more research on each one as needed.
Internet Explorer
The Browser Helper Objects are registered under
HKLM\Software\Microsoft\Windows\CurrentVersion\explorer\Browser Helper Objects
IE toolbars are registered under
HKLM\Software\Microsoft\Internet Explorer\Toolbar
Chrome
Storage Location for Packed Extensions
Navigate to chrome://version/ and look for Profile Path, it is your default directory and Extensions Folder is where all the extensions, apps, themes are stored
Windows
If my Profile Path is
%userprofile%\AppData\Local\Google\Chrome\User Data\Default
then my storage directory is:
C:\Users\<Your_User_Name>\AppData\Local\Google\Chrome\User Data\Default\Extensions
Linux
~/.config/google-chrome/Default/Extensions/
MacOS
~/Library/Application\ Support/Google/Chrome/Default/Extensions
Chromium
~/.config/chromium/Default/Extensions
Thus for each browser there will be a different file structure or on Microsoft browsers probably Registry key layout.
Custom code all the way...