Soldered RAM is what it is.

You can’t match what you can’t change. Soldered RAM is fixed to the board. You don’t get to “match” its timings or voltage — you work around it.

If your laptop has one soldered stick and one free slot, the system will run both at the slower stick’s speed and timings. So buy the fastest stick the soldered RAM supports — or just grab the same spec from the manufacturer’s documentation (search your model + “RAM specs”). Voltage isn’t negotiable either; most modern laptops run DDR4 or DDR5 at 1.2V or 1.1V respectively, and the slot will pull whatever it needs.

If you’re trying to replace soldered RAM entirely? Don’t. It’s not coming out without a rework station and a steady hand. Buy a laptop with more RAM upfront next time.

Don’t overthink it. Check the sticker on the existing stick (if visible) or look up the model number. Otherwise, buy a common speed like 3200MHz for DDR4 or 5600MHz for DDR5 and let the motherboard sort it out. It will.