What is the role of an examiner in a undergraduate defence (license) knowing that the jury is made up of a rapporteur who is both president, examiner and both academic and professional supervisors? This is my first experience as an examiner. I am in Tunisia.
As far as I know, Tunisian customs in academia are similar to French ones. For example, all the roles you have listed correspond exactly to what one would find in a French defense jury. I will thus answer what I know about French customs.
The role of an examiner (examinateur in French) is to be present at the defense, to ask questions of the candidate during/after the defense, and to participate in the deliberations after the defense that lead to giving the grade and/or writing the defense report when applicable. Unlike the referees (rapporteurs), it is not typically expected that they have read the thesis in detail before the defense. The choice of examiner can occur late in the process and examiners are not usually involved in supervising the candidate - that's the role of the supervisors/co-supervisors. They can even be external to the university, although that's uncommon at the undergraduate level. An examiner can be president of the jury as that role is largely ceremonial (deciding who speaks and when).