Course player templates can be found in these folders:
The main templates that are used in the course player:
masterstudy-lms-learning-management-system/_core/stm-lms-templates/course-player
Templates related to lesson pages:
masterstudy-lms-learning-management-system/_core/stm-lms-templates/course-player/content/lesson
Templates related to quiz pages:
masterstudy-lms-learning-management-system/_core/stm-lms-templates/course-player/content/quiz
Templates related to quiz questions:
masterstudy-lms-learning-management-management-system/_core/stm-lms-templates/course-player/content/quiz/questions
Lesson templates and drip content:
masterstudy-lms-learning-management-system-pro/stm-lms-templates/course-player