The present perfectThe aspect expressing completion. E.g.: He’s finished university. tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses. is used for recently completeA concept, related to the perfect aspect, expressing that an action has finished. E.g.: He’s finished university.d actionA general term expressing that somebody or something does somethings, usually with the word just.

I've just bought a processor. My computer needed to be upgraded.
I have just started a typing course.

Note that when the exact point of timeA particular moment in time. E.g.: She came home at six. ("six") I was born in 1980. ("1980") is mentioned we use the past simple.

I bought this processor two weeks ago. My computer needed to be upgraded.

Just now also requires the use of the past simple tense and means "a moment ago".

I spoke to him just now. He says hi.

Diagrams

Present perfect to express annoyance
Present perfect Present perfect with FOR and SINCE