How to swap usb charging mode to file transfer mode (ftp) on Android 7.0?

On nexus 6p

Welcome to Android Central. To connect to a PC and transfer files (not FTP) you have to enable Developer Options on the phone and install appropriate USB drivers on the PC.

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Then chose MTP mode (there's no FTP mode).

If you want to use FTP, you'll have to install an FTP server (see https://play.google.com/store/apps/c...1ljL-jGM&hl=en ).