If I want to set up a vpn tunnel in a network to securely transfer files between android devices connected to the network , what can i do ?
A kali os is responsible for destributing public, private keys.

Tried to use wireguard for that but the apk wont support file transfer.

Thanks

The idea of a VPN is not file transfer.
You could just use a plain SSH-server for each device (as I assume the Android devices aren't rooted, you'd have to run them on unprivileged ports) and use SFTP.

Otherwise, can't you just access a central storage server (Samba, or even FTP) and access it via some apps?

Maybe Nextcloud/Owncloud on a central storage would also be a solution. There are many tools for Android which support Webdav. Depending on what you want to achieve, maybe you can just sync with that.

Openvpn + nextcloud running on the server, and clients just connect to it via clients?
Or an sftp server/client setup instead of nextcloud