Basic question regarding references

Hi, I am thinking of applying for a PhD. On the website of the PhD offer, it was mentioned that I need to provide the names and contact details of three referees.

Will these referees be asked to send a reference letter to the person(s) evaluating my PhD application? Or will they be interviewed or something? I don't know what to expect, because when applying for my masters my referees were asked to upload their reference letters on a website. The PhD website doesn't mention anything about this.