Ubuntu 18.04 not detecting full RAM size

This line:

[    0.000000] Memory: 6929972K/7238032K available (12300K kernel code, 2472K rwdata, 4248K rodata, 2408K init, 2416K bss, 308060K reserved, 0K cma-reserved)

indicates that BIOS is giving the kernel 7238032K of memory. The rest of the memory is being used by the BIOS:

8,388,608 K Total - 7,238,032 K Kernel = 1,150,576 K reserved by BIOS.

That is an unusually large amount for BIOS to reserve. You would have to look at your BIOS settings to perhaps try to figure out why.

From: Why my acer nitro 5 (AN515-42, Radeon RX 560X) 8gb ram but its just 6.9gb ram usable?

This can only be "fixed" in the bios! But not all bios allow you to change the pre-allocated memory of the video card, because there is a risk of the notebook turning a brick! Can you take a picture of this setting?