I have tried all suggested solutions for this on the web, but none of them work in Ubuntu 18.04. Need to remap my Ctrl + C and Ctrl + V to CMD + C and CMD + V, as well as tilde character.
Did you try this here ? :
Sure, that USB-port is really under voltage ?! Or is USB-port somehow not active ?!
Plug in and out keyboard usb and what delivers command lsusb ?!
Normally usb is plug-and-play , but it can help when you reboot your machine after keyboard is plugged in.
Two different points are being conflated in this question, AFAICS.
 Yes, Ubuntu 18.04 does have English (UK, Macintosh) in its list. You can see it in the screenshot here: https://linoxide.com/linux-how-to/configure-keyboard-ubuntu/
I am running Ubuntu on a 2008 Macbook at home with the Mac layout.
You can add both and switch between them: https://linuxconfig.org/how-to-add-and-switch-keyboard-layout-on-ubuntu-18-04-bionic-beaver-linux
 However, this will not automatically redefine keys for you. Mac keyboards have both Control (Ctrl) and Cmd (Command) keys. "Cmd" is equivalent to the "Windows" key on modern PC keyboards, or what very old-timers call the "Super" key.
On a Mac running Ubuntu, or an Ubuntu PC with a Mac keyboard, Ctrl-C is still Ctrl-C. If you want macOS-style keystrokes (Cmd-C to copy, etc.) then you must remap Ctrl to Cmd and (I would suggest) Cmd to Ctrl.
That is described here: How to remap Super(left) key to control key
N.B. If you are using GNOME, then the easiest method by far is in the last answer, right at the bottom.