Liam,
First off you’re mistaken. While United (and the other Majors) all want a 4yr degree it need not be in aviation or related field (Mentor Chris is a United Capt and I believe he majored in business admin or something like that). As for the how’s I recommend you spend some time browsing this sight (this thread in particular https://airlinepilot.life/t/how-do-i-become-an-airline-pilot/10891).
Adam