Soldering RAM onto empty pads won't work without BIOS support.

Probably not.

Even if the motherboard has empty solder pads for a second RAM chip, the memory controller and BIOS need to be configured to recognize it. Laptop makers often use the same PCB for different SKUs — the lower-end model just leaves the pads unpopulated and disables dual-channel support in firmware. Dropping a chip on there won’t magically enable it; you’d need to flash a modified BIOS (if one even exists) and possibly change resistor straps. Plus, soldering a BGA package like LPDDR RAM isn’t a DIY job.

Even if you get the hardware right, the laptop might boot, but only see 8GB total, or fail to train memory entirely. Unless you’re comfortable reverse-engineering firmware and have a rework station, this is not a practical upgrade.

Buy a laptop with 16GB from the start.