Investment banks use internships as extended interviews. Your technical knowledge matters, but the broader question is whether the team would trust you as a full-time analyst with work under time pressure, sensitive client situations, and long or unpredictable days. That is the real test.
The evaluation process varies by bank, office, and group. Some firms use formal mid-internship and final reviews, while others rely more heavily on informal feedback from the analysts, associates, vice presidents, and senior bankers who worked with you. Staffing levels, business needs, and firm performance can also affect hiring decisions. You cannot control every factor, but you can make the performance case for hiring you as clear as possible.