How to express about getting informed about a specific position
I don't see any reason to express where you learned about a job advertisement, except if it was emailed personally to from whoever you learned about it, and not widely distributed. However, usually these are mass emails sent to a large number of people (recognizable by several factors). In that sense, I don't see any reason why you would need to state how you learned about the position - unless you want to make a point out of it, i.e. make it clear to Prof. X who advertised the job that their good friend Prof. Y (or maybe the very famous Prof. Z) suggested to you personally that you should apply, which hopefully reflects positively on you in the eyes of Prof. X.