Soldered RAM is not upgradable via BIOS, custom or otherwise.

No. This is strictly hardware-limited. The RAM chips are physically soldered to the motherboard. No amount of BIOS tweaking can add more chips or change the ones that are already there.

A custom BIOS could hypothetically let you reconfigure memory timings or enable undocumented features, but it cannot increase capacity. The physical memory lanes and controller are hardwired to the installed chips. Linux doesn’t change this either — the OS just sees what’s there.

If you need more RAM, buy a laptop with sockets next time. Or look for a model that has an empty slot in addition to soldered RAM. That’s your only real upgrade path.

Save yourself the headache. Soldered means soldered.