I applied to the CSAP and participated to the 3 selections phases for the Switzerland role.
Due to Covid-19, I did every step from home, through the Webex application.
First, a classical phone call : Nothing much but assessing the motivations of the candidate by asking them to adress any questions. The trick here is that if you don't have any, the conversation won't go far and can become awkard quite fast.
Then, 2 weeks later, I was asked to answer questions (12) by recording myself at home : 3 attempts per questions, 3 minutes max. Not very hard but be prepared to answer questions like : "Can you tell me about the you faced ...?".
Lastly the Assessement Center, taking place virtually (Covid-19 style). 2 hours of : interview / Roleplay / Presentation on a given topic. You have more or less 1 week to prepare. The difficulty here is not so much technical (if you are still in the race you have the level and time to reach the desired knowledge), but to be able to swith from one interviewer to the other, stay focus and cool.
During the whole process, the people you will talk to will emphatize a lot on the will to give you feedback and see how you implement it during the tests. So ask for it as much as you can.
But do not fool yourself, as soon as you are not selected, communications stops and you just get a standard 'thank you for trying' e-mail.
Still, you will learn a lot during the procedure, and even if I did not make the cut, it was of great help for the next interviews I did.