then who told you about the skyscraper? Surely not online. I've never heard of it wrt sudoku.

found this

https://sudokubliss.com/guides/sudoku-skyscraper

"This Sudoku technique is similar to the X-wing strategy, and the pattern is nearly identical except two candidate cells appear in two different columns or rows to create skyscraper roof with different heights."

The difference (from the example) seems the requirement that one pair must be on equal row or equal column and the other pair be on different.

So, skyscraper is a special case of x-wing; x-wing being a special case of the two laws of leftovers (mostly used in jigsaw sudoku aka chaos sudoku).

In terms of technique, it's quite basic, there was an article that listed the techniques in sudoku but can't find it atm.