Will Apple's 90-day warranty cover a cracked LCD on a MacBook Pro?

The 90-day warranty is only on the parts that were replaced, in your case that's just the battery.

If your MBP is still covered by the 1-year orginal warranty the screen may be covered; if you are beyond 1 year but have Applecare+ then the cracked screen could be replaced but you will have a co-pay of at least $99.

Apple standard warranty covers defect in materials or workmanship for the standard warranty period. in the US, that period is one year.

Regarding items made of glass:

if it survived being shipped halfway around the world from the factory in China and was delivered to to you without obvious visible defeats, Apple assumes it did not have defects when delivered to you. For that reason, glass breakage is NOT considered a defect under standard warranty.

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if you want coverage for glass breakage and many other items, the AppleCare Plus extended warranty is a great idea. Co-payments are required, but for many repairs, those co-payments are quite reasonable. You MUST purchase within 60 days of original receipt of your Mac, and if not purchased when you purchased you Mac, it MAY be subject to inspection.

The reason you must purchase promptly is that, if you could purchase at any time, only users with problems would purchase, and that would ruin the "insurance effect" and make the coverage impossibly expensive.

Italianstein099 Said:

"Replacement: I just had my battery replaced on my MacBook Pro 2020 (4 thunderbolt chargers) no other damage was found at all. I did this at the Apple Store. Then within the 90 day period while holding my laptop the lcd cracked on the computer will that be part of their 90 day warranty since it’s in 90 days or because it’s a different part it will not be the same."

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Getting Items Replaced:

No. Even with AppleCare, there is a price to pay for getting the screen replaced. As it rads here: AppleCare Products - Mac - Apple: "each subject to a service fee of $99 for screen damage or external enclosure damage". But you only have 60 days to get AppleCare. As for getting the battery replaced, you have a one-year, out-of-the-box, limited-warranty. If it were free of charge, then it would have to be seen as a factory defect. These are just user-to-user forums. Not one here has any call in returns and replacements. So, take this in for a Genius Bar Appointment at your local Apple Store. There, it will be diagnosed, and you'll be provided with your options.