The drive uses the M.2 2280 form factor, which makes it 22 mm wide and 80 mm long.
Like most M.2 NVMe SSDs, the HP EX900 Pro connects to the host system over a PCI-Express 3.0 x4 interface.
On the PCB, you'll find the controller, four flash chips, and one DRAM chip.
Chip Component Analysis
The Silicon Motion SM2263EN is a four-channel flash controller with support for NVMe 1.3, TLC, and DDR3/DDR4 memory. Apparently, HP has customized it in some way, as evident by the HP logo and the different model number.
The four flash chips are Intel/Micron 96-layer 3D TLC NAND; they have been rebranded, too. Each chip has a capacity of 256 GB.
A Micron DDR3-1600 chip provides 1 GB of fast DRAM storage for the controller to store the mapping tables.