It's not the verb "have/has" that is used in the Pr.Perf.Tense. It's an auxiliary verb here and serves to form this Tense together with the past participle of the verb "to be-been" and the verb "to finish-finished". The Present Perfect Tense denotes an action that happened in the past with the result at present. In sentence #a you state that you visited America in the past but by the moment of speaking it turns out that you have been there a lot of times during your life.
As for sentence #b: you finished working on your paper some time ago-so you can show it ready to be handed in, for example.
The use of so-called "signal" words, like "just", "already", "yet", "ever', '"never" normally indicates the Pr.Perf. Tense. As well as the following phrases: "all my life", "since we last did smth", after "how long" when asking for a precise answer, as in: "How long have you known her?"
All the best,