The I-20 is the official document that your university sends
you after you have been admitted there and have completed all the
admission steps.
Once you receive your I-20, you can apply for your visa. Since Corona, embassies also accept a printed copy of the I-20.
Here
you can see an example of the 1st page of an I-20 with the data you
will be asked for in the visa application. In the original I-20, your
personal information is entered at the top. You have to sign the I-20 on
the 1st page (do not fill in anything else) and send it back to the
university by email.