What is the Universitaly pre enrolment, step by step?

Short answer: The free official online form that links you, your university and the Italian embassy. Every non EU student who needs a visa must do it (MUR circular).

Create an account on Universitaly with your own email. VFS Dhaka will use that email to contact you (Embassy notice).
Fill in the pre enrolment for the one course you will attend, and choose the embassy for your country of residence.
Upload what the university asks for, often your passport and admission letter (Tor Vergata).
Wait for the university to validate it.
Download the summary. The Dhaka checklist wants it to show your enrolment and tuition fees.

Clearing the confusion: you can apply to many universities, but with most foreign qualifications you pre enrol on Universitaly for only one course, and you cannot later enrol at a different university or course (University of Pisa).

This page explains official rules in plain language. I am not an immigration adviser and I do not give visa advice. For that, always use official government sources. Rules change: confirm with your university, the embassy and VFS before you act.