I use 2 methods for this:
if I need to find a specific activity type/class in a huge project:
→ look up the activity type in the activity list, right click, find references. A search result will show each file that contains this activity type.
The other one, if I for example need an actifity containing a specific value or variable which is not easily found with the internal search:
I use notepad++ to scan my entire project folder for the keyword I need. It will also yield a search result with all relevant files. (ctrl+shif+f → find in files)
So search all *.xaml files on the value “xxx yyy zzzz” and you know exacly which files contain it.
The latter option requires a bit more ‘finesse’, but works if done right