Since the Samsung Galaxy S6 has limited internal memory, you have to go and free memory space. In general, you connect the smartphone via USB to the computer and then transfer, for example, videos from the internal memory to the PC.
Especially with videos it can happen to you that they do not appear in the "DCIM / Camera" folder on your computer. This is due to an setting of the Android system: To recognize video files in MP4 format on the internal memory of the Samsung Galaxy S6 on your computer, the following transmission mode must be selected: "Media"
This you can adjust as follows:
If your Samsung Galaxy S6 is connected to the PC via USB, pull down the status bar and tap on the alert for "USB connection". You can choose now how to connect. The choices are:
Set the mark at "Media ". Then, the Samsung Galaxy S6 reconnects again with the PC. You will then find the corresponding video files in the DCIM / Camera folder.