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I only use C-x 1, C-x 2 and C-x 3, seems like that covers 95% of my use cases at least.

C-x 0 can also be useful.

Never used it, I tend to use C-x 1 to get back to one window; what's the difference?

No difference if you have two windows, but if you have three or more you just kill the current one giving the space to one of the adjacent windows.