Doubts in present perfect tense?

I give two conversations here below:

a. Here is some context.
Mary is talking to Tom on the phone. She has been trying to get in touch with him for the past two hours.
M: Where are you? (Present of 'be'.)
T: I'm at home. (Present of 'be'.)
M: I've been trying to reach you for the past two hours! Where have you been? (Present perfect continuous of 'try' then present perfect of 'be').
T: Oh! I've been out in the garden and I've just come indoors. Sorry about that. (Present perfects of 'be' and 'come'.)

b. Here is a conversation between me and my friend.
me: Where are you?
my friend: I am in the car.
me: how long have you been in the car.
friend: I have been in the car for 3 hours.
me: What are you doing in the car? just sitting or driving?
friend: Why? I am driving the car.
me: How long have you been driving the car?
friend: I have been driving the car for 3 hours.

Now my question is below:

"Present perfect" links past and present some way.

1. Could you tell me please, how does present perfect link past and present in the cases of "a." and "b."?