On a different tab in the same spreadsheet? (It may work on different spreadsheets, but I haven't tried it.) Same way as you would on the same tab of the spreadsheet (well, one of the same ways that you could...).
In the destination cell, enter '=', navigate over to the source cell and click to select that cell. Done. (Unless you want to apply a formula to that value...that's obvious too, but I always check that I'm getting what I expected before I complicate things further, but the 'cell colouring/debug functions don't really help that much when you can't see both cells at once, obviously.)
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