It was both, but they bootlooped for different reasons. Nexus 5 had a faulty power button that could fail in a couple of ways. Nexus 5X was something about how the components on the PCB were constructed being of iffy quality. You could get lucky and never have the 5 fail whereas the 5X was/is guaranteed to fail. I bought a 5X purely for device testing. It was basically unused 99% of its lifetime (6+ years) and eventually started bootlooping last year.

The Nexus 6P was also famous for having a bootloop issue as well as suddenly going dead despite showing that it still had a good charge.

Both my wife and I had these and, despite initial promise, they behaved horribly. One particular issue was overheating and shutting down. IIRC the boot loop issue, when it eventually struck, was worked around by a hack to disable two CPU cores. Due to this and the poor performance of Wear OS (or was it still Android Wear at that time?) we switched to iPhones. I note that the reason I first got a 6P was because my 6 completely died one day and a 6P was the only available insurance replacement. Despite all this, I'm likely to by the Pixel 8 and Pixel Watch 2 to replace my iPhone 8 plus.

Yes, it was a Nexus 5. There's been several phones that have had this issue from Google... which is all the more reason I wouldn't buy another Google hardware device.

I had a Nexus 5X that bootlooped, but if IIRC from my research at the time, Nexus 5 also had the same problem, albeit seemingly to a lesser extent.