It all depends on what type of game you are working on or what the job description is you are applying to. You could be designing combat and AI or you could be working on the story and assets for a level. No matter what you work on you will breakdown how your idea will work in the game and what will be needed to achieve your idea.
Check out Gamedev map(https://gamedevmap.com/) and check out what certain companies are looking for in video game designers. I would also find your favorite game companies and contact some of the people working as designers to see what their day to day consists of. Don’t expect everyone to answer you back but if you show that you are interested in becoming a designer in the future, some will be willing to give you information.