There was a little bit of a discussion about how fwhm is calculated here:
https://skyline.ms/announcements/home/support/thread.view?rowId=54862

The FWHM value is determined by finding the two points farthest from each other within the integration boundaries where the background-subtracted intensity on the chromatogram is at least half of the background-subtracted height at the apex of the peak. Here is a page which shows how Skyline calculates the background level of an integrated peak:
https://skyline.ms/wiki/home/software/Skyline/page.view?name=tip_peak_calc

If you have questions about "FWB" (full width at base), you might be asking how Skyline decides where to place the peak boundaries themselves. I do not know of anything which talks about exactly how Skyline decides where exactly to put those peak boundaries.

There is a bit more information about peak finding in section 3.3 of Dr Pino's "Skyline ecosystem" paper:
https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/full/10.1002/mas.21540
but I don't think that paper ever talks about how Skyline decides where to put the peak boundaries.
-- Nick

Thanks Nick