When does the interview start...
The interview starts the moment you hit ENTER on a job application.
Landing a job requires skills that are COMPLETELY DIFFERENT than the skills required to actually DO the job.
You may be the best person for the position, but it is a matter of marketing that to the Hiring Managers. The Hiring Manager has to be able to SEE you in the role.
Remember, they don't know you yet. This is the only way they can try to determine if you are the right fit, so everything counts.
Everything you put forward is part of your "interview", showcasing who you are, the quality of your work, and what your values are.
What you post on social media, your LinkedIn profile, the effort put forth in your resume and cover letter, your response time to emails, professional attire, how polite you are to everyone, not just the person interviewing you...
And most importantly, how you conduct yourself during an interview and if you have the ability to give examples and tell your stories, which would convince the potential employer that you are the best person for the job.