I am unable to understand the use of ~ here.I know that find peaks returns two arguments x and y axis of peak

Answer 1

[Stephen23]

https://www.mathworks.com/help/matlab/matlab_prog/ignore-function-outputs.html

In both of the examples you show, the code's author wanted the second output from FINDPEAKS, and so ignored the first output using the tilde. What outputs a function has is explained by its documentation.

"I know that find peaks returns two arguments x and y axis of peak"

The findpeaks documentation actually says that its first two outputs are:

1. "pks ... a vector with the local maxima (peaks) of the input signal vector, data."
2. "locs ... the indices at which the peaks occur."