When your folder structure and mounts are in place you install the base system into the new partitions.
The minimum required is the base meta package.
But to actually run the install we need kernel, a tool to create the initial image, network, boot loader, boot manager, text editor and sudo.
Run basestrap command to install the packages into the newly mounted root
# basestrap /mnt base linux-latest dhcpcd networkmanager grub mkinitcpio efibootmgr nano sudo