The hiring process for any Big 4, not just EY, is truly rigorous. There are many requirements and suggested steps to be met alon the way. For example, recruitment for Big 4 internships begins the spring of your junior year. Here, you will apply for summer leadership programs. These programs are designed to introduce candidates to the firms professionals and various service lines. It's a great way to get your foot in the door but highly competitive so be prepared to interview and bring it.
After summer leadership conferences, there is summer recruiting events that are typically informal and sometimes invite only. They can range from being very small scale with a few professionals and yourself to a larger BBQ social type event. The more successful candidates are proactive and set up coffee/lunch meetings with professionals of the various firms.
After the summer, fall of your senior year is when internship recruiting begins. You apply through your Universities career services (usually online) and if selected, you are scheduled for an on-campus interview. The firms all interview generally within the same week so schedule your interviews so they aren't back to back (looks bad on your part). On top of this week long series of interviews (one per firm) you will have nightly social events (not mandatory, but don't shoot yourself in the foot). It is very busy. Not to mention it's generally during your first round of midterms!
Next, if you survive first round interviews, firms will reach out and extend a second round interview to you. This is where, regardless of where you are going to school, the firm will fly you out and put you in a hotel, all expenses paid, for whichever office you are recruiting with. You will have three, 30 minute interviews back-to-back-to-back (drink water!) usually with senior managers and partners. Then later that night typically there is a social event. Repeat this process for as many Big 4 or large regional firms as you are interviewing with.
You will receive your offers after this point. Some firms even offer you early, prior to second round interviews to try and get you early before you are persuaded by the other firms. It's competitive and firms want the best candidates. Bring your A game!
And, if you don't get an internship, you can go through full time recruiting the next fall, where you will start as a Staff 1. Same process as above with first and second round interviews. There can be exceptions though (i.e. if you are no longer on a college campus and an experienced hire). Generally speaking, firms will hire first out of their internship programs to fill spots before hiring directly for full time. Internships are great "trial runs" where the firm can evaluate a candidate's fit with the firm and ability prior to offering a full time position.