NVMe SSD upgrade suggestions please.

Hi, I just bought a Razer 17 and I would like to upgrade the storage to the maximum (8TB). The specifications say this: "Gen4 NVMe upgradeable to 4TB + Open M.2 Slot (Supports 2-sided NVMe Drive), upgradeable to 4TB SSD" I guess that means the first slot can only take a single sided SSD. I can't find this info on a lot of product pages. I'm also confused about heatsinks and whether I need to install one with them. Basically, I would like to know exactly which products I can put in each slot. Links to the product pages would be appreciated. Not concerned about price and the faster the better. Thanks!