Your SRM data file has two chromatograms with Q1=104.07 and Q3=58.196.
When Skyline reads the chromatograms from the data file, Skyline will only use one of those two chromatograms, and will ignore the other one.
Here is a page that talks about how Skyline does background subtraction:
Skyline does not do any sort of curve fitting when calculating peak areas. The points along the chromatogram do get interpolated in the retention time dimension, but that has a very small effect on the peak area. The "Skyline Ecosystem" paper talks a little bit about this resampling in the section "B. Resampling":
I hope that answers your question. Let us know if you have any other questions.