I need to access the internal sotorage I was changing the ROM on my phone (Samsung Galaxy S5 SM G900P) when I deleted the OS. Now I know I can flash a new one by adding a zip file to the SD card or Internal storage.

The problem is that I can access either one of those since all i can go into is recovery and download which doesn't enable the usage of File transfers via USB. I need to know some way for me to get access to the storage, any help is welcomed.
In this you have one option of sideload in recovery. In this case you need custom recovery. If you don't have a custom recovery them you need to flash it using odin in download.
Now boot in recovery mode. Go to install > Install from sideload (in cwm) or advance > sideload (in twrp).
Now when you open it, open terminal / command prompt. Type command 
adb sideload your_rom.zip

It may seem that nothing is happening. You must have to wait and don't touch usb until work is done. Any interruption may hard brick your device. Even though it is safe. 


  For this you must have required adb tools ( for this you can search ).