For my notes, click my username and via private message send me the PC service tag number.
It is "possible" to install the HDD since we sell it with both SSD and HDD. Here is the online Inspiron G5 15-5587 Service Manual. Page 32 discusses HDD. If you do not buy it with an HDD, you would need to purchase T0GN3 HDD interposer + cable. The problem is that our parts page shows this part as not available to sell. I doubt that Dell would include T0GN3 if a HDD is not pre-installed by us. My advice, buy one with both the SSD and HDD.
I tried telephone support and was severely frustrated. I could NOT understand the gal, she spoke too fast and it sounded like she was answering about RAM on the M.2 but not sure, as I said, her accent was heavy and I could not understand. So used chat and had much better experience. But, was told 1 TB was maximum. Is that your take, also?
2TB is the largest HDD we used = NNFJ4 5.4K RPM, 512e, SATA3, 2.5 inch, Rosewood7-2D
1TB is the largest SSD we used = X10G4 PCIe-34, M.2, 22mm/80mm/2.15mm, NVMe, 3D V4 NAND