How should I discuss about a paper with my potential supervisor who is one of the authors of that paper? Last week I met my mentor (and also my potential supervisor) for my PhD. He suggested me to read a paper which is also related to my master's thesis.

I read that paper and some questions popped into my mind and I want to share them with him. Also, I would like to impress him and show him I fully understand the paper. I wonder how I should talk about the paper with my potential supervisor.
Organise a meeting with him, sit down and say "Bob, I have a couple of questions about that paper you suggested I read last week". And then ask the questions.

Don't overthink this. It really is that simple.