D1 and D2 schools issue nli but you have to receive athletic scholarship money to receive one. The actual letter is sent to the student a day or so effort the opening of the signing period for that sport and in the fall the students has one week to sign and return. Some do it in a high school signing ceremony but you don’t have to. You can receive more than one but can only sign one letter. There is also an award letter that comes with it.
most athletes are admitted, but some are not. Wisconsin did not admit a recruit for football this year even though he was ncaa eligible. My daughter’s coach had admissions look at the scores and grades of the recruits before she made offers. Not binding, but why waste anyone’s time if the recruit isn’t going to be admitted?