I have a question about - The selection procedure

No, the written test is on a set period of days. 
For the oral test at Enabel's (part 1: motivation interview, and part 2: group exercise and  competence-oriented interview), candidates can choose from among several dates (via Doodle) that have been e-mailed to them. The candidates can choose the date that suits them best.

No, you must be present in Belgium to do the selection tests.

Yes, you do. The Junior Programme recruits candidates from a variety of backgrounds, also so-called 'non-technical' profiles.
Certain job openings are for 'soft' project aspects like raising awareness or mainstreaming gender, communication, or monitoring and evaluation.

Have a look at the examples of tasks of former or current Junior Experts on the website