iPhone 12 Pro Max. Screen glitchy and black color pixilated

It seems your issue isn't a black screen that is not working at all, it is simply the quality of the display and the functioning of the keyboard.

See if simply restarting the phone clears up the issue. If not, take it to a Genius Bar if you can find one, or contact Apple. I also presume you purchased this from Apple and if you are not happy with a product you can return it in a two week period. You could then buy a different one.

Read this support article to learn how to restart your iPhone --> Restart your iPhone - Apple Support

How to make a Genius Bar appointment

1. Go to Genius Bar Reservation and Apple Support Options - Apple

2. Go to "Get hardware help".

3. Select your hardware and continue selecting options requiring replacing parts until you see "Bring in for Repair"

4. Sign in with your Apple ID

or:

Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone ➞ Contact Apple for support and service - Apple Support

Click here to read about Apple Store Returns & Refunds--> Returns & Refunds - Shopping Help - Apple - For the USA "You have 14 calendar days to return an item from the date you received it." The return terms may be different in some countries (the link I provide should open to a page for the country to which your account is set).

- "You can always return the item to any U.S. Apple Store. An Apple Specialist will assist you with your return. Your refund will be processed at that time"

Here are some common solutions you can attempt:

1. Follow the Apple article to resolve the issue --> If your iPhone, iPad, or iPod touch won't turn on or is frozen ...
2. If not resolved; please follow the steps in the following link for support with this issue: If your iOS device restarts or displays the Apple logo or a spinning gear unexpectedly
3. Sometimes, a simple restart can resolve temporary software glitches. Turn off your iPhone, wait for a few seconds, and then turn it back on.
   1. Restart the device. That's it.
   2. Still not solved --> Force restart iPhone - Apple Support
4. Make sure you have the latest updates installed. Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)
5. You still see a black screen after charging, force restarting, following the steps in this article.
   1. You see a black screen, but you can hear alerts, sounds, and vibrations.
   2. The screen remains black, but the device chirps when you unmute it and connect it to power.
   3. Your screen is on, but it doesn't respond when you tap, slide, or try other gestures. 
   4. You can't force your device to restart because a button is broken or stuck.
   5. Your device is stuck on the Apple logo, displays a solid color, or isn't recognized in iTunes.
   6. Contact Apple Support to set up service if you have any of the above issues:
6. Please follow the steps in the following Apple Video for support with this issue:

If none of the above steps resolve the issue, it is recommended to reach out to Apple Support or visit an authorized service center for further assistance. They can provide specific guidance based on your device model and iOS version.