Xcode. device is not connected.

When Xcode said  your device was not connected but actually it was connected.
it would be solved reboot however avoiding reboot, type the below command.

sudo launchctl stop com.apple.usbmuxd

stackexchange

What is launchd

according to wikipedia,

launchd is an init and operating system service management daemon created by Apple Inc. as part of macOS to replace its BSD-style init and SystemStarter. There have been efforts to port launchd to FreeBSD and derived systems.

it could solve other problems too..