Sharing my setup on using 3.5" HDD with PS4. This is done for the larger capacity and faster RPM. The HDD here is a 2 TB SSHD from Seagate.
The green wire is a 7 pin Sata data cable. Male to Female.
This enables you to extend the connector inside the PS4 out to the HDD. The air vent holes at the back is just nice for letting it come out while still allowing me to close the plastic cover.
The power source comes from this 12v AC adapter that outputs to a 4pin molex. This is part of a kit that was meant for people to connect their HDD through USB to their PC. So it also comes with a 4pin molex to Sata power adapter.
Then you follow the helpful guide from SONY itself on upgrading the HDD.
SONY PS4 HDD Upgrade instructions
Level of difficulty is kinda low, does not involve any modding, complicated tools, other than a screwdriver, mostly is 'Tab A into Slot B' thingy.
As an extra upgrade, I will be putting a HDD cooler that draws power through a molex connector bypass. Something like S$4 from eBay.