The answers so far are not entirely correct. The warranty does not expire in every case. If you are still able to load the stock ROM after a defect, it will be repaired by the manufacturer under warranty (of course, only if other warranty conditions are met). I had such a case when the DA module for the sound failed. This was repaired under warranty without any problems. At that time, I was still able to flash back from Cyanogen to stock.

Of course, it must not be a defect that was demonstrably caused by an app that uses root. Then, understandably, it will not be repaired under warranty. However, this happens very rarely.

Yes, as soon as the bootloader is overwritten, you lose the warranty.