Can you give some details about the process to apply for EPSO jobs like assessors, trainers, coaches?
Maybe I am misunderstanding your question and you are referring to people working as assessors, trainers and coaches at EU Training/Arboreus. This is probably up to them to reply to this question. From what I can tell, some of the these assessors, trainers and/or coaches have themselves been involved in competitions such as Mr Jan De Sutter who was once the President of a Selection Board for an EPSO competition. Others may have been working in a similar HR, psychology and cognitive testing related work environment. After all, the EPSO competition system is not completely different from other similar competitions used at some larger companies in the private sector or in some countries' public sector.
People "working at EPSO" as assessors are usually permanent staff from EU institutions that apply and are selected to fulfil this role for the purpose of competitions. There are also some EU staff that are seconded to be assessors for EU competitions on a long-term basis for a few years. There are no trainers or coaches from EPSO as such to help people prepare for these competitions. There are various training offers from private companies such as EU Training/Arboreurs and also training offers from EU trade unions or EU Member State public administration career services that work together with trainers and coaches who have (some) experience with EPSO competitions.