You can browse memory of a camera, phone or other devices only if they are mounted as external drives (option on some cameras and phones). Most cameras and phones use PTP and MTP protocols which is not supported by OC, and support is not planned because of following reason:
MTP protocol is very unreliable and can lead to data loss. I personally have lost data several times using Windows File Explorer - see the difference in folder list of my phone when accessing through FTP wireless or when accessing through USB with MTP protocol:

You can see that 4/11 folders are not even listed, and would miss the several folders to back up. That doesn't happen consistently which is even worse. In addition, MTP protocol doesn't allow multiple operation at the same time, so you can't copy and browse folders on Android device at the same time, which would lead to freezing of UI.
The main thing is that I wouldn't want to be responsible for people loosing their data, even if the protocol is faulty, so that is the reason I decided not to implement it. In addition it is hard due to lack of documentation on it. I had other developers try to implement it for OC but without success, and if Microsoft cannot properly implement it, then I would not be able to do it either.
I personally use FileZilla to access files remotely on my Android phone that runs FTP server. There are many free apps that have FTP functionality but I found "
Primitive ftpd" to be the best for me. It supports concurrent tasks and have not had issues with it in the last 4 years. Maybe someday I will implement FTP in OC, but I for now I suggest using
FileZilla.