You can't disable soldered RAM in BIOS.

No, not that I’ve ever seen. BIOS/firmware settings for memory are usually limited to things like XMP profiles, timings, or maybe disabling an entire memory channel if the board supports it — but not individually disabling a soldered module while leaving the socketed slot active.

The reason is simple: soldered RAM is part of the motherboard’s memory routing. The system sees it as a fixed part of the memory controller. Even if the BIOS offers a “memory configuration” menu, it’s not designed to let you pick and choose which physical sticks to ignore. You can sometimes disable a memory channel (if you have multiple channels), but that would also disable the socketed slot if they share that channel.

If your goal is better performance, mixing soldered and socketed RAM can actually hinder it — you might lose dual-channel mode, or the system might run at the slower speed or loose timings of the soldered module. If you want to use only socketed RAM for performance, you need a board that has only socketed slots. Or buy a laptop with no soldered memory.

Best case: check if your specific motherboard has a “memory channel” enable/disable option in advanced BIOS settings. Even then, you’d likely need to disable the entire channel, which might also kill the socketed slot. Not a clean path.

Save yourself the headache — if you need to control your RAM, buy a machine with fully replaceable memory.