Re: how do i read/view app cache on samsung s5?
The cache is a collection of bits and pieces that the app uses while it's running - it isn't useful data once the app stops. (It's like reconstructing a restaurant's menu by throwing the contents of its dumpster into a large food processor, then going through the mush.) You're not going to find messages in the cache - you might find pieces of messages mixed in with numbers (not phone numbers but things like the length of a particular string, or the size of an icon), picture data, etc. It's a temporary trash can, where only the program knows what's where, and only while it's running. Close the program and open it again and it disregards the current contents of the cache and starts over. Once the app closes, the cache is just a bunch of meaningless bytes. (Which is why you can clear the cache on any app that's not running without losing anything.)
If you want to save messages (I don't use FB Messenger, so I don't know how it's set up), you have to use the app's "save messages" function (if there is one).
Facebook
Twitter
Instagram