Match capacities or lose the benefit.

No.

Adding a socketed RAM stick to a soldered one will not always run in dual-channel mode. It only runs fully dual-channel if the capacities match exactly. Even then, the memory controller has to support flex mode (Intel) or asynchronous dual-channel (AMD), but that’s the standard for modern laptops.

Here’s how it actually works:
Say your laptop has 8GB soldered. If you add another 8GB stick, you get full dual-channel for the first 8GB of address space. The last 8GB (soldered + new) will run single-channel, but it’s symmetrical enough that performance is fine.

If you add a 16GB stick to an 8GB soldered module, you get dual-channel for the first 8GB of each channel (so 8GB from the soldered + 8GB from the SODIMM), and the remaining 8GB on the SODIMM runs in single-channel. You still get a speed boost for the first half, but not for all memory.

If you add a 4GB stick to an 8GB soldered module? You get single-channel for the entire address space, or maybe