I used "CPU-Z" on my new Inspiron 7779 and it said I had 2 (two) memory slots. Currently only one is populated with a 16 GB DIMM. Given that there are two slots, that does beg the question of whether the 7779 will support an additional 16GB DIMM.
The documentation on the 7779, sparse though it may be, indicates that 16GB is the largest supported memory configuration.
So .. can I add additional RAM to the 7779?
The internal training page also states 16GB maximum (2133MHz, DDR4 dual channel, sodimm). One 821PJ 16GB sodimm or two MKYF9 8GB sodimm. Some user will need to test another 16GB in the empty slot. Boot into the BIOS and see if it shows 32GB. Or it may not boot at all. Then we will all know the non-Dell tested capabilities.
If you decide to take the back off the unit, be careful. I found a latch which is not shown in the manuals and is not easy to get to release.