Unlock iPhone X while screen is broken and won't turn on?
You can try this slightly unconventional approach. It would work easily if you can get your hands on an alternate working iPhone X, the same model as your iPhone X.

Place the other working iPhone X besides yours. Now based on the screen of working iPhone X, you can approximate the location of the on-screen controls.

First, attempt to turn off your iPhone X. Use the help of the working iPhone to locate the power-off slider. After you have turned off your iPhone, turn it back on. You'll be prompted to enter the device passcode. Use the working iPhone X to approximate the location of buttons and unlock your iPhone.

If this is the first time you are connecting your iPhone X to your Mac, you'll need to trust each other. Connect both your and the other iPhone X (which I am assuming was never connected to your Mac). You will get the approximate location of the Trust button on your iPhone.

Once you have successfully connected your iPhone, you can back it up in iTunes.