Everything you've said sounds like a wire connected to one of Teensy's pins, or the 3.3V power, accidentally touched 5V power.
I see you posted photos in msg #8. In msg #3 you said "it looks just like the board photos shown throughout the tutorials". But your photo looks nothing like the tutorials, where the Teensy has pins soldered and it's placed into a solderless breadboard. The photos on msg #8 show a Teensy 4.0 without pins, with all pins in pristine unsoldered condition.
My best guess is you were attempting to do the tutorials with clip leads and loose parts, rather than using a solderless breadboard. While that can work, and from everything you've said it was working until you got to tutorial #3, with clip leads and loose parts all the materials are so light weight and move so easily when you pull on any wire or the USB cable. It's a recipe for wires and parts accidentally touching each other or other places on the Teensy.
The damage to your Teensy 4.0 is consistent with a pin or 3.3V accidentally touching the 5V power on VIN and VUSB. The pins are not 5V tolerant. Touching them to the 5V power accidentally, even for just a fraction of a second, destroys the main chip.
From the tone of every message you've written, it seems you do not believe you could have made this mistake. But if you really were doing those tutorials using loose parts connected to the Teensy by clip leads, even if you used extremely careful measures like securing every wire to the table with tape, the odds of any of those clip leads accidentally touching the wrong place is very high.
The bad news here is measuring 0.5V on the 3.3V line while nothing else is connected to Teensy means your hardware got damaged. We've seen this before many times. As Frank said (yes, somewhat abruptly) it doesn't happen spontaneously for no reason. This damange happens when pins or 3.3V power are shorted to 5V or higher power supplies, or negative voltage is applied (rare - but it happens sometimes when people do audio projects with opamps using positive and negative power supples).
I understand it's painful, and expensive, to have this sort of thing happen. But at this point, the only viable path is to replace the Teensy. If you do buy another one, please consider getting it with the pins already soldered (unless you have a soldering iron and good soldering skill) and get the solderless breadboard. The breadboard is solid and relatively heavy, so it doesn't move and flop around like clip leads connected to loose parts. It also holds all the parts in place, so if you do move the cable or accidentally move it around, all the parts move together. The odds of wires accidentally shorting to unintended places is greatly reduced... not as good as a printed circuit board or other hard soldered construction, but solderless breadboard construction is a very reliable way to prototype circuits. Please, do yourself a favor and use a solderless breadboard on your next attempt. I'm sure it will work out much better for you.