It is private to the app. Mapillary does not want you to have access to your own photos. And the security model of Android explicitly considers the phone owner and user as a threat and therefore gives the apps the opportunity to hide data from them.
Only if you have rooted your phone, you will be able to access that storage.
I read somewhere in this forum that the mapillary app might re-enable accessnat some pont, but since about 2020 there has been no change.
see lots of threads, e.g. Cannot access Mapillary photo storage on Android device via USB