The thing that is most important to me when interviewing new hires is that they understand the backgrounds.

Honestly for a trainee or junior position you can’t expect a lot of skills or proven work. Brush up your knowledge, learn about the backgrounds of the algorithm, learn about how bidding works for different campaign formats.

Also be yourself, don’t bullshit and admit to not knowing if that’s the case. In most good agencies I know entries are not hired by skill but rather by personal fit and potential. So present yourself as someone eager to learn. Show that you can comprehend complex systems and communicate that you are motivated and eager to learn.