Hello,
After my Raspberry Pi 4 failed booting I noticed some loose wires in micro SD card slot as shown on a photo. It seems to be a problem. Why and what happened to the Raspberry Pi? Should I apply the warranty?

No, those two wires touch each other when a SD card is inserted. It's a kind of switch that tells the Pi if it should boot from there or not.
It is the card detection switch but it isn't connected to anything on any of the Pi boards. The firmware detects card presence by trying to talk to it.