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There's a MicroSD slot on the Surface Pro 7. It's underneath the kickstand so it's protected. No need to open anything to add a chip. It clicks right in. I added a 512 Gigabyte chip in that slot, and it shows up as a D drive. Not every software application will load to this drive, but some will. And it's great for picture, video, and file storage, of course. Cost for a 512Gig chip was less than $80.
Depending on the version, you can add a flash memory card to most Surfaces.
Yes you can expand the memory by using an external hard drive or a USB attached to the computer port.