Does an M.2/NVMe SSD use PCIe lanes from the north bridge, or the south bridge?

[Freesurfer]

I have a 1TB SATA drive that I bought some time ago, but it is constantly full and I need to shuffle things in and out to free up space.
I am wondering whether to add in more capacity with extra/bigger SATA drives, or getting an NVMe drive, but without robbing PCIe lanes from the GPU.

My system is built from an Asus Z170A motherboard, i7-6700k CPU, Nvidia RTX 2080 GPU, 16GB RAM. To my knowledge the system has 20 PCIe lanes, out of which 16 are used by the GPU. I don't know about the remaining 4 lanes and if they may be assigned to things like on-board USB. It has one USB-C slot, as well as a teal USB type A port labelled "USB 3.1", as well as a bunch of USB 3.0 slots and other I/O like audio and 1000Mbit ethernet. I don't know if these may somehow use any of the PCIe lanes.
Then there's the empty M.2 slot, which I think should support an NVMe drive. When I looked into it some time ago, it had the correct slot keying to accept an NVMe drive. And the manual stated something along the lines of "SATA or PCIe mode". The term "NVMe" is not mentioned specifically, but this motherboard is 5+ years old and NVMe is a relatively new term, I guess from before the industry properly settled on NVMe.

But back to the question.. considering that the GPU uses 16 lanes and only 4 are left.. Is there a risk that installing an NVMe drive will drop the GPU to only 8 lanes?

[thm_w]

See these threads:
https://pcpartpicker.com/forums/topic/187715-asus-z170a-m2-support
https://linustechtips.com/topic/691758-z170-motherboard-3rd-pcie-slot/

Should come from the chipset, unless you use a PCIE card instead of the m.2 slot.