The simple past can refer to a past time that by definition excludes the present. (I say can because we also use the past tense form to refer to the present in some cases.)
The present perfect refers to a period of time that began in the past but continues right up to the moment of speaking.
The activity or event indicated by the verb does not have to continue up to the present moment, but it can. What is the case is that the past activity is relevant to something observable at the moment of speaking.
I have fought in a war.
This is fitting because the fact that I've fought in a war has continuing significance or meaning to me. The experience makes up part of who I am in the present. I know what war is like.
It is up to the speaker to clarify (if he chooses) whether the activity is still going on at the moment of speaking. The statement 'I have fought in a war' often implies that the speaker is not still fighting in that same war. This is true for most dynamic or action verbs. But the context of the statement helps to determine the issue.
As such, the present perfect can imply that the past action not only began in the past, but that it is still going on. 'I've studied English for ten years' certainly implies that one is still studying English when the statement is made (not literally at that exact moment, but at "an undefined ongoing time that includes the present").
'I've practiced safe sex all my life, so don't worry' certainly implies that the speaker is still in the habit of practicing safe sex at the moment of uttering the statement. But one never knows for sure.
'I've worked at that company'
This means that I worked for that company in the past, and that this past action has some relevance at the moment of speaking. Depending on the context, it may or may not imply that the speaker is still working there at the the moment of speaking.
But if someone would ask me why I am so sure about ominious experiments conducted by one company, I reckon my answer would go:
I know it, because I used to work for that company.
Yes, this is a natural construction to use here. The "used to" indicates an ongoing action in the past that is not still going on in the present. The very fact that you "used to" work there gives you knowledge about the experiments (at least this is what you claim.)
You could also use the simple past: "I know it, because I worked there for x years." Again the action of working there for x years happened in the past. But that experience gives you knowledge about the experiments (or that is what you claim). However, the use of the simple past means you are just reporting on an event that was completed in the past.
But you could just as well use the present perfect:
"I know because I've worked there for x years and I am sure some ominous experiments are being done." Here the use of the present perfect with for x years indicates that the speaker still works there at the moment of speaking.
You could also just say
"I know because I've worked there and I am sure some ominous experiments are being done."
In this example, the hearer cannot be absolutely sure whether the speaker still works there or not. The context of the statement can help determine this, as well as the hearer's knowledge about the speaker. This shows that the use of the present perfect (and any verb form) depends on the viewpoint of the speaker. And as I said way above, the speaker does not have to clarify if the past action is still going on at the moment of speaking. Language allows for a lot of ambiguity.
Note: I wrote an answer about the present perfect today also on ELU. It gives more examples.