The HDD can be replaced quite easily. 1TB 970Pro comes to mind, to boost the overall performance quite significantly, as I have done, compared to the Western Digital HDD that my laptop came with. I know that HP uses various HDD manufacturers - sometimes you get Samsung OEM SSD HDD, most of the times you do not, which is a shame for the price they charge. WD lags in performance, even when compared to Samsung OEM PM961 and PM981.
Also, the RAM is most of the time Samsung RAM, but sometimes they use Nanya chips - again, resulting in a performance hit.
The WiFi card is firmly soldered to the PCB / motherboard.
The CPU can be "repasted" very easily with a good heatsink compound. I used arctic silver, resulting in around 4-5 deg C less heat.
Overall, it is a good laptop, in particular -> its sturdy built. Just make sure you'll be getting Samsung SSD and RAM, if you decide to buy one.
This is the third one we bought for us. All running with HP docks with zero issues.