this is no concrete request for help but rather a generic question:
When installing the Android file sync client, you can go the Android settings → accounts → (Your NC account name). There you have the option to enable a toggle for Files.
What does this setting control? I do not understand the use case here. Can you explain this to me?
Nextcloud usually synchronizes the list of available files and creates a virtual file list, so that you can see which of them are avaiable on the server. Once you selected a file it will be marked with a tick mark and synchronized to your device so that it is still available if you are not connected to the server (aka offline files). All local files can be seen/checked using the “on my device” menu entry.