I added the WD SN850 drive to mine. You need a PCIe Gen4 x4 NVMe drive. If you've added a NVMe drive to your pc, then you're half way there. The hardest part for me was getting the cover off my PS5. Damn thing didn't want to come off. There are tons of videos on youtube that show you how to do it. Here's one from Sony:
Even with the issue I had with the cover, It was back on the shelf in 20 minutes. Be sure to upgrade the firmware to the PS5 first, and you might want to get a drive that comes with a heatsink. I got a drive without a heatsink, and got the heatsink separately, because I couldn't find a drive that came with the heatsink in stock. This youtube site tested quite a few drives for use in the PS5.
Just so you know, external drives on the PS5 are only for PS4 games.