"built into the mobo"
I seriously doubt that. I've taken hundreds of laptops apart. It's likely just hidden and not accessible to the average user.
Onboard memory is common when it's a tablet but not a laptop.
I've seen many times where 1 dimm is under the keyboard (or hidden elsewhere) and the other is on the bottom side with an easy access panel for upgrading.
In either case laptop memory is notorious for being finiky on the install. When you slide it in (while it's still angled up), press left and right on the corners of the DIMM and then press down to lock it into place. Often laptop memory needs to be wiggled or pushed in really well before you snap it down into place.
ASUS said that it does accept an 8GB RAM stick, but they said that after they said that it couldn't be updated. This was ot the only reason taht I think I was talking to an amateur.
They told me that it would be a 8GB DDR3L 1600 SDRAM.
The RAM stick that Crucial told me to put in it is an 8GB DDR3L 1866 SODIMM.
I have learned that a higher frequency doesn't matter because the system will just resort to the lowest frequency anyway, but....
...I'm not sure about the differences between SDRAM and SODIMM. I think the support person wasn't very clear on these things either.
Crucial said to update the BIOS and VGA. I got the BIOS updated, and that didn't improve things. I downloaded the VGA driver update too, but I'm unfamiliar about how to get it to install through the Linux Peppermint OS.