I guess this is an exercise from school ? you could find all the answers on internet.
Q1- the system can located the user thanks to the Location Update procedure. The system knows the “Location Area” of each user.
Q2- phones don’t ring all at the same time because when someone calls someone else, then only the “called” phone is going to ring. Not all the phones in the netwok. The called phone knows that it is being called thanks to the Paging Request message (on the BCCH), that contains its own and unique TMSI. The Paging message is sent from all the cells in the current Location Area.
Only the called phone will answer to this specfic paging.
Q3- when two users talk simult. then nothing special happens : both users can hear and talk at the same time (one timeslot per direction and per user)
Q4 – as secure as the A5/1 algorithm is. Which means : not much. (you can find “A5/1” in wikipedia.
Q5- key components ? you mean… the main elements ? like MSC, BSC, and so on ? you can find this in wikipedia, by searching “GSM”. You’ll have all elements, and their functions.
I hope it was brief enough 🙂