After researching the company and at least knowing a few things about the ad tech space typically someone will reach out to the company. (To stand out I would suggest stopping by in person unannounced looking sharp and dropping off your resume). After submitting your resume and there is an opening and you look like a serious candidate they will email you back to set up a phone interview. If that goes well they will invite you in for one or possibly two interviews with a director of the department you are looking to get into and possibly with one of the co-founders. Depending on how you do they will email you an offer and you go from there.