getting replacement PCIe-NVMe SSDs

In the past when I bought laptop's I'd get the smallest / cheapest (SATA) HDD available and then swap it out with a better one from newegg.com.

This one particular laptop I'm now looking at comes with a PCIe-NVMe SSD. My question is... how interchangeable are those? They're not soldered onto the motherboard, are they?

When I do a search on newegg.com for "SSD PCIe-NVMe" I get cards that are obviously for desktop's, motherboards and cards that appear to be for laptop's. There are two interfaces: PCI-Express 3.0 and PCI-Express 3.0 x4. There are 4x form factors: 2.5", AIC, M.2 and M.2 2280. How do I know which of these I need? Are some of these form factors / interfaces for laptop's only?

The laptop in question is a ThinkPad P50.

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