Warranties can be tied to limitations and restrictions the manufacturers make up. (There probably are some limitations to that to do with contract law. It surely is quite complicated, but voiding your warranty by unlocking your device doesn’t seem unreasonable to me.)
Warranties are voluntary and distinct from seller liability for defects in the product. That liability is legally required in the EU. The details of the implementation are different in different countries.
In Germany it’s like this: The seller (and only the seller) is liable and you have to go to the seller to make a liability claim. So if you bought your phone from Amazon you have to write Amazon, not Motorola. If you bought it from a carrier you have to go to the carrier. If you didn’t buy the phone directly from the manufacturer that manufacturer doesn’t have to do anything when you come to them with a defect phone. It’s the seller’s problem. (This is mostly to prevent a runaround, where seller and manufacturer both tell you each other is responsible for fixing the problem. This creates one entity that is clearly and obviously responsible and has to handle the problem.)
It’s for defects present when the device was sold and that’s it. (However, subsequent defects of some component because the device was delivered with some faulty or not up to spec components count, too.)
In the first six months the seller has to prove the defect wasn’t present when the device was delivered to not be liable. You, the buyer, don’t have to prove anything. That burden of proof, however, reverses after six months and up to 24 months (then there is no more liability), making it very hard in practice for buyers to prove after those six months that the defect was there when the device was sold. (However, courts have relaxed the requirements for that. Buyers usually don’t need to get some expensive expert opinion or something like that. If you can make a very good and informed argument for your case you should usually be covered.)
The seller can repair or replace the device. Buyers don’t have right to get their device replaced when it’s possible to repair it. If repairing and replacing fails a number of times (I think three) the buyer can demand their money back.
That’s the liability. A legal requirement and very complicated – and also wholly distinct from warranties.
By unlocking your phone you may have voided your warranty, but you certainly didn’t void the legal liability of the seller.