Brigitte,
Assign roles, aka role override, can only be done at the activity level. Sections don't have this ability.
It sounds like you want to have the trainer create new activities themselves in their section, so you or a teacher don't have to make them first, but the permission for that can only be set at the whole course level (Manage activitiesmoodle/course:manageactivities).
I think the "Moodle way" to do something like this would be to create a separate course for the Trainer and make them the teacher in it. Then you could look into using either the Subcourse plugin https://moodle.org/plugins/mod_subcourse and/or Meta courses to join them together, https://docs.moodle.org/310/en/Course_meta_link.
Assign roles, aka role override, can only be done at the activity level. Sections don't have this ability.
It sounds like you want to have the trainer create new activities themselves in their section, so you or a teacher don't have to make them first, but the permission for that can only be set at the whole course level (Manage activitiesmoodle/course:manageactivities).
I think the "Moodle way" to do something like this would be to create a separate course for the Trainer and make them the teacher in it. Then you could look into using either the Subcourse plugin https://moodle.org/plugins/mod_subcourse and/or Meta courses to join them together, https://docs.moodle.org/310/en/Course_meta_link.