Thanks for the reply: and I did say I'm a non-specialist user I don't really know the way Zotero works.
I'm just going by what is on the Zotero site.

(1) I WOULD like to understand Zotero better;
Could you please give me a link that might explain things better?
I searched before I asked my question.
Perhaps what's there needs a bit of a re-write?

(2) However, your reply did not answer my example. If Zotero can access metadata directly from Sciencedirect, then why doesn't it access the example I gave?

The articles in my example all have DOI's.
In fact the files are LABELLED by the DOI.

And why I thought it involved a Translator, was that the Zotero documentation says translators are what Zotero uses to extract Metadata from websites.
That's what it says here:
https://www.zotero.org/support/adding_items_to_zotero

So surely, Elsevier/ScienceDirect must have a pretty consistent set-up for Zotero to retrieve MetaData from?

I can give you another example:
V/Stol Airplanes by McCormick
I search on Sciencedirect, and it finds it easily:
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/takeoff

And here's its DOI:
https://www.sciencedirect.com/science/article/pii/B012227410500911X

And again, maybe my jargon is not right, but surely if the article has a DOI, and it's easy to find, then why can't that be used by Zotero to retrieve MetraData?

(3) There DOES seem to be a bug in Zotero about how it accepts DOI's:
Please read ALL my step:
- I add the article in pdf form into Zotero
- It says "No matching references found" (which is how my question started)

So I then try to use a DOI:

Method A:
I right click on the pdf file, and select "Create Parent Item"
It then gives me two choices:
- Manual Entry
- "Enter a DOI etc etc"
BUT if I try to use the DOI here, Zotero gives an error:
"Lookup Failed: Zotero could not find any identifiers in your input."

So that stopped me.
However, I see on reading more on using DOI's in Zotero you can input it another way:

Method B:
Toolbar on Zotero with Wand: "Add items by identifier"
I then enter EXACTLY the same as I did in Method A...and it WORKS.
It FINDS the article.
Great.

But Method A has a bug in it.

Hi Adam,
And firstly, thanks for your usual patience in answering questions.

But no, Method A doesn't work for me.
Specifically, when I am at the pdf in the main pane of Zotero, and Right Click.
I get various options, from:
View PDF
Show File
Show in Library
...
Create Parent Item
Reindex Item
Update Citations

So I click on "Create Parent Item", which gives me an entry sub-window with:
area to input data
"Manual Entry", "Cancel", Create Parent Item" (greyed out until you enter some characters in the data entry spot)

A few issues:
I CANNOT use Paste from my mouse to enter data there.
I also cannot use Ctrl-V/Ctrl/C from, keybourd to copy/paste the DOI info.
It ONLY accepts manual typing in.

And THEN when I type in the correct DOI (https// etc etc etc)....it THEN says "Trump is the Emperor.....you are forked".
OK, I joke.
It says "nothing found".

I'd send you screen shots but I think this forum too basic, yeah?

HOWEVER, when I use the option at tool bar, it works (Method B).

So look, no big deal, I figured it out.
But there definitely IS a bug in Zotero.

I'm on a Mac, High Sierra.
Zootero 5.0.95.3

If you want to follow it up, tell me what you need.
But from my end it seems unless I gear up to to use JavaScript better I'll leave it as it is.

However, I see that as of 2019 there's a plug-in for DOI's: I'll try that.

Alan
P.S. I am not actually a COMPLETE idiot....a few parts are missing.

@WarthogARJ: We've fixed the right-click menu in the Create Parent Item dialog in Zotero 5.0.96, available now.