Specs:
One 3.5-inch hard drive
One M.2 2230/2280 solid-state drive
One 3.5-inch hard drive plus one M.2 2230/2280 solid-state drive

You only have one NVME slot for an SSD. So a second SSD would have to be a SATA SSD but it would have to replace the HDD, if you have a HDD now. The one you linked is NVME and that slot is already taken.

Thanks.  My reading of the "supports one of these configurations" didn't seem to imply a 2nd drive could even be added.  So, with a SATA SSD, could you point me to installation instructions?

What's the purpose for the 2nd drive (SSD)?

If you don't have an HDD now and just need more storage space would you do better installing a 1- or 2-TB HDD? Might be less expensive than a SATA SSD of the same capacity.  And you can always make Win 10 move its default folders onto a secondary drive to free up space on the boot NVME SSD.

This computer shipped with two empty 2.5-inch hard-drive cages and one SATA extension cable. SATA SSDs are 2.5-inch so you'd mount the SSD in one of those 2 cages. You'll need the SATA data cable to connect to the motherboard and a SATA power connection to the power supply.

The Service Manual gives instructions to install a 3.5-inch HDD, so the procedure would be similar for an SSD, except you'd mount the SSD in one of the 2.5-inch drive cages. NOTE: Some 2.5-inch SSDs come with an adapter tray to install it in a 3.5-inch HDD drive bay, so you might use that bay with an adapter tray if it's easier and there's no HDD in there now.

Is that cable connected anywhere?

SATA data and power cables are separate. SATA HDDs and SATA SSDs have separate/different connectors for data and power.

In most cases, Dell doesn't include an extra SATA data cable. The manual doesn't say whether drives installed in this model get power from the motherboard using one of Dell's customized power cables or directly from the PSU.

So I can't answer your questions. Search these forums to see what others have done to install additional drives in this PC model. You can't be the first...

Thank you for your kind help.  It's getting clearer I think:)  It looks like there is only a data cable inside that reaches to the 2.5" cages.  Is that maybe power and data, or is there another cable I haven't seen?  Or does one come with the drive?

Am I understanding that even though we have a empty 3.5" sata hdd rack and two empty 2.5" sata rack, I can only add one more hard drive?  I couldn't add a 3.5" sata hdd and also a sata ssd in the 2.5" with adapter or for that matter two sata ssds in the two slots?  

Thanks

There are three sata connections on the motherboard and only one is being used by the hdd that came with the PC. I assume then up to three can be used if they have plugs for it.

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This post from last year addresses what you're trying to do. It seems the G5 is configured similarly to the Vostro 5890 I purchased last year: SATA power and data cables provided to the 3.5-inch slot, and a SATA power cable with two connectors (and no data cables) for the pair of 2.5-inch slots. Presumably, if your system is configured like the other poster's (and the picture in the manual), it came with blue plastic mounting trays in each of the 2.5-inch slots. If you wish to install a 2.5-inch SATA drive in one of the two slots, you'll just need to acquire a SATA data cable as well, and run it to any available SATA connector on the motherboard.