Candidates are selected over the period of a week in the middle of March every year. Potential supervisors at CERN can browse the CERN website during this time to consider the completed applications and select their student online. There is no test involved for candidates.
No, technical and administrative students receive a monthly subsistence allowance from CERN to cover the expenses of a single person living in the Geneva area.
In your application form you have the possibility to indicate your fields of interest. However, you cannot choose the subject. Selected technical and administrative students are always assigned to a specific project with a CERN supervisor and the student can then decide to accept or to refuse the offer. You can also use this placement to do your final project (Bachelor or Master thesis).
There are two Committee meetings per year, usually in June and December. You can check the individual programme description to check the upcoming committee dates.
CERN is not an educational establishment and therefore does not give out any study credits. However, in the past, supervisors have certified the attendance at lectures of his/her student and this has been accepted by some universities.