It looks like your favo(u)rites (certainly for 2018 at least) will be stored here:
C:\Users\%username%\AppData\Roaming\Autodesk\Inventor 2018\Content Center\Settings\Favorites.xml
Edit: I just did a quick test between Inventor 2018 and Inventor 2019 and the results were not entirely surprising.
This is a diff between the 2018/2019 versions of the file above:
https://www.diffchecker.com/lplmFFxL
If you look near the bottom you will see that the only part which is the same for the BS 4168 family is the NodeId; the InternalName, LibraryId are different. In my case I believe this is because I have copied this particular family to a custom library in order to allow me to add a different material to it.
The reason I can tell this is because the library this came from - "Inventor Other" - it's InternalName is unchanged from the previous version:
old: InternalName="271cc27b-14af-45c8-8686-bc036143cde5"
new: InternalName="271cc27b-14af-45c8-8686-bc036143cde5"
In terms of being to copy old favo(u)rites to Inventor 2019, I'm not sure this will be possible, and it'll certainly be a manual process unless someone from Autodesk has another solution?
