Vertical orientation and slot numbers like on the picture below:

Assuming you are counting the flexbay position while the machine is lying down horizontally.  Slot 3 and 1 will accept SATA drives.  Slot 4 and 2 will accept NVMe drives via carrier.

If the machine is standing vertically, top 2 slots are for NVMe drives, bottom 2 slots are for SATA drives

3 and 4 of flexbay 1 are for NVMe drives, backplane connects to dual minisas HD on motherboard.

1 and 2 of flexbay 0 are for SATA drives, backplane connects to SATA ports 0 and 1.

When you said drive not detection, assuming you meant they did not show up in BIOS.  Then reset BIOS as the default value enable all SATA drives and both PCIe0 and PCIe1 on VMD controller.

Chino, thanks a lot. According to 7920 Quick Guide: https://www.delltechnologies.com/asset/en-us/products/workstations/technical-support/Precision_7920_Tower_Technical_Guidebook.pdf

„FlexBay 1 is default for 1st and 2nd M.2/U.2 PCIe NVMe SSDs”

Also here: https://www.delltechnologies.com/asset/en-us/products/workstations/technical-support/Precision_7820_Tower_Spec_Sheet.pdf

„NVMe PCIe SSDs are supported in FlexBay 1 on PCIe backplane chassis with integrated Intel controller…”

Similar thread: https://www.dell.com/community/Precision-Fixed-Workstations/HDD-Slot-not-working-in-7920/td-p/7800848

https://www.dell.com/community/Precision-Fixed-Workstations/Dell-Tower-5820-Additional-SATA-SSD-not-recognized/td-p/7849641

If you only get one carrier for flexbay 1 and wanting to add a second drive, consider Icy Dock U.2 adapter.

I have got original Dell M.2 PCIe SSD carriers: one from mentioned 7820 and one from 5820.

Regarding SAS HDD carriers I noticed that pin distance is different on 7820 (left) and 5820 (right):

Goal was to detect all four drives but first I need to check it in UEFI BIOS F2 or F12 Boot option drive list using an original configuration:

• FlexBay 1 backplane slot 3/4: M.2 SSD
• FlexBay 0 backplane slot 1/2: SAS HDD

I think both of the carriers are the same.  You can move the little blue rubber which are acting as hard drive screws depending on the drive's size.  Your picture shows the left one was setup for 2.5" drive while the right one set for 3.5", hence the spacing looks different.

The integrated controllers are for M.2/U.2 and SATA.  If you want SAS, you will need a tri-mode controller card such as MegaRaid 9440-8i or 9460-16i.

https://www.dell.com/community/Precision-Fixed-Workstations/Installing-NVMe-SSD-in-Precision-5820-FlexBay/td-p/6089597/page/2

Carriers are almost the same: black plastic distance pins (top right corner of each caddy) have different length.

Regarding M.2 SSD PCIe NVMe slots according to T7820 documentation: metal hook to protect/route cables is on the bottom (like black sockets for M.2 SSD carrier) and wire harness connectors are on the top of the motherboard so M.2 SSD carriers shall be installed in bottom slots (Flexbay0):

A bit of questions :

was it sold without the OS installed onto it ?

you sure that the top flexbays are converted for u.2/nvme usage ?

Since I suspect a bit of confusion, can you provide a photo of the actual unit showing the flexbay area taken from the front ?