Can confirm. Happened to me with my old laptop. Tried to upgrade it with some rescued RAM and it refused to use all of it. It would only use up to the laptops advertised max.

(Not the brand in question, but motherboards can definitely limit the RAM utilisation)

If you are looking for a distro similar to Arch where you can compile everything from source you can just use Arch. https://wiki.archlinux.org/title/Arch_build_system

Disclaimer: I have never used this feature so don’t know how easy it is to use, but I do use Arch which is how I know it exists. (I use Arch btw)