Between Present Perfect and Simple Past, which tense indicates a finished action?

When one uses simple past tense for some action, does it imply that the action is complete?

For instance, when you say “I wrote my article yesterday,” does this imply that at this moment you have a finished article or not? So it’s

I wrote a bit of my article yesterday, but I still have to finish it.

vs.

I started and finished writing my article yesterday and now I have a finished work, so I can publish it.

What about present perfect? Does it imply completeness? If you say “I have watched this film,” does this mean that you have actually watched it from start to end?

If both these tenses imply that the action was completed, do I have to use past continuous to emphasize the incompleteness of action? For instance, “I was watching this film,” so that the implied meaning is that I started watching it, but it’s not certain that I finished it.

when you say “I wrote my article yesterday,” does this imply that at this moment you have a finished article or not?

Yes, you finished it. Otherwise, as you say, you would say something like "I started to write my article yesterday". In that case, 'started to write' has finished even if the article hasn't.

What about present perfect? Does it imply completeness?

No, it doesn't. Present Perfect is 'until now' or 'relevant now'. It says nothing about what happens after now, though context might. For example:

"I have worked in that factory" implies (by using 'that') that you no longer work there. Maybe somebody is asking you what is inside, and you have relevant knowledge because you have worked inside the building.

"I have worked in this factory for ten years", but this is your last day. You have worked here until now.

"I have worked in this factory for ten years", so you know where the toilets are and don't need telling. Unless you get sacked you will continue to work here.

do I have to use past continuous to emphasize the incompleteness of action?

If the action is incomplete, you use present continuous. Past continuous is for a finished action.

In your example “I was watching this film”, the action - the watching - is complete. You are not watching it any more. Whether you completed the film or not doesn't really come into it, as the film is not the action.

I largely agree with Roaring Fish. Let me add a few comments too long to fit as comments.

In general, the simple past tense indicates that the action is complete. But grammatically, it just means that the action indicated by the verb is complete, not necessarily that the task as a logical concept is complete. Like if you say, "I watched this film", that means that your action of watching is complete. But that doesn't necessarily mean that you watched the entire film. You might have walked out halfway through because it was boring or offensive or whatever. But either way your action of watching is done.

If someone said, "I wrote this article yesterday", I would normally guess that they mean that the article is complete. But grammatically, that is not necessarily so. It would be perfectly reasonable to say, "I wrote this article yesterday. Would you please review it and I will make updates based on your comments." That is, the article is not complete, it is just a first draft. (People trying to be evasive often play games with the grammar. Like the politician who carefully says, "I am not having an affair with that woman", because he declared the affair ended yesterday, and so he is not having an affair in the present.)