Thx for the graphical feedback. Appreciated ! I guess this is as easy as it can get for hardened pc builders, but for a newbie like me, it's still akin to Chinese*:D.
Basically, what I'd be interested to know is what would happen if I plugged the GPU on either of the 3 16x PCIe lanes.
According to the mobo manual, the breakdown is as following :

3rd Gen AMD RyzenTM Processors
- 2 x PCIe 4.0 x16 SafeSlots (supports x16, x8/x8)
2nd Gen AMD RyzenTM Processors
- 2 x PCIe 3.0 x16 SafeSlots (supports x16, x8/x8)
2nd and 1st Gen AMD RyzenTM with RadeonTM Vega Graphics Processors
- 1 x PCIe 3.0 x16 SafeSlot (supports x8)
AMD X570 chipset
- 1 x PCIe 4.0 x16 (supports x4)
- 1 x PCIe 4.0 x1

First two (from CPU) x16 assuming you leave the one not in use empty or else changes to x8 & x8 with direct connection to the CPU
Last one is x4 (don't use for your graphics card) and is connected to x570 chipset.

Ideally drop your graphics card in the first one as this is least likely to cause you an issue and where it really should be.

Generation of Ryzen changes pcie 4.0 or 3.0 specifications. Most people don't purchase x570 without ryzen 3000 series CPU (and 4000 series when they get here), which is gen 4.0 pcie. All older ryzen CPU would pcie 3.0 specification, i.e ryzen 2000/1000 series which no one would buy for this motherboard anyway.
x570 chipset is pcie 4.0 specification.

Personally I find the graphics easier than the manual.

Anything else you need to know or need clarification on ?

Thanks.
I thought indeed putting the GPU in the first x16 slot, leaving the second one empty. That gives me a full x16 potential right ?
The x1 PCIe slot doesn't interfere with anything else, is that correct ?*
Concerning the 1 x PCIe 4.0 x16 (supports x4) : why isn't it a full x16 one ?
I'm supposed to get full PCIe 4.0 potential for my two M.2 SSDs right ? What I didn't quite get is how the PCIe slots could interfere with this.
Same question for the SATAs. Can I plug them into any of the SATA connectors or do some share bandwidth with certain PCIe slots?*

irepublicofgamers wrote:
I thought indeed putting the GPU in the first x16 slot, leaving the second one empty. That gives me a full x16 potential right ?

Yes.

irepublicofgamers wrote:
Concerning the 1 x PCIe 4.0 x16 (supports x4) : why isn't it a full x16 one ?

Connected to the x570 chipset that is why and link from x570 to the CPU is the limiting factor. Same for the x1 one.

irepublicofgamers wrote:
I'm supposed to get full PCIe 4.0 potential for my two M.2 SSDs right ? What I didn't quite get is how the PCIe slots could interfere with this.

The limiting factor is the pcie between the x570 chipset and the CPU. The link between them is x4 wide.
If your asking in RAID 0 then yes you get full potential. (although you lose all data if strip 0 crashes or unsync's and don't recommend them)
If your asking using benchmarks than first slot will always do better as it has direct connection to the CPU
If you're using this second drive as say game drive and the first for OS / office / documents then you never notice anything is different.

Apart from tests both m2 drives are extremely fast but if both drive are identical and no other factors are in play then first drive directly connected to the CPU is faster than the second because for the second to talk it must pass through the x570 to the CPU and longer distance and never going to have zero other communications to deal with.

Dont assume to much, get your system up n running, it will be faster than you think. The specifications dont tell the whole story.

What CPU do you have?

first drive directly connected to the CPU is faster than the second

ok so in that case it's better to install OS and apps on the higher M.2 slot (closest to CPU) and documents to the second one, right?
*

What CPU do you have?

*
ryzen 9 3950X

Any specific thoughts on the SATA SSDs? I believe there's 6 slots.*

irepublicofgamers wrote:
ok so in that case it's better to install OS and apps on the higher M.2 slot (closest to CPU) and documents to the second one, right?

Personally I would use the second for games and long term storage because documents are not that big and the way OneDrive works / lazy cant be bothered changing that.

irepublicofgamers wrote:
Any specific thoughts on the SATA SSDs? I believe there's 6 slots.*

x570 controls them, and they share the link to the CPU as per the diagrams.
If you need sata, than add what you need.

Here is the procedure for install of windows on nvme drive.

1 - Make sure you unplug all SATA and USB drives, the M.2 drive has to be the only drive installed.
2 - Go into the bios, under the boot tab there is an option for CSM, make sure it is disabled.
3 - Click on secure boot option below and make sure it is set to other OS, Not windows UEFI.
4 - Click on key management and clear secure boot keys.
5 - Insert a USB memory stick with a UEFI bootable ISO of Windows 10 on it.
6 - Press F10 to save, exit and reboot.
7 - Windows will now start installing to your NVME drive as it has its own NVME driver built in, I don't think this would work with previous versions.
8 - When the PC reboots hit F2 to go back into the BIOS, you will see under boot priority that Windows boot manager now lists your NVME drive.
9 - Click on secure boot again but now set it to Windows UEFI mode.
10 - Click on key management and install default secure boot keys
11 - Press F10 to save and exit and windows will finish the installation.

Once you have Windows up and running, shutdown the PC and than reconnect your other SATA drives.