I am going to try and answer this inline as you cover much much more than you think your do, and you lack of understanding of email protocols may well bite you.

I'm trying to understand the difference between archive and export and copy so I can get my task done with the least confusion. I have 24 hours experience with TB. Yesterday I downloaded the software and successfully added 2 email accounts. They are infrequently used accounts so I can afford to experiment before adding my primary email account. My primary email account is web-based Outlook. I'm bumping up against the maximum storage before needing to purchase more storage or deleting emails.

So an IMAP configured account which is synchronized to the server is really not going to be much use to you without a true export option as offered by the import export add-on, but it will probably be slow. IMAP is also what Thunderbird created unless you chose to change the default in the new account to POP

Add-on https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/?src=ss but search for it by name in the add-on manger (Menu> add-ons and themes) to make the installation simpler.

If possible, I want to copy entire folders from the primary account to local folders on my PC using TB.

Not a good choice as silent timeouts tend to see mail just go missing. So while technically possible I do not recommend it.

I thought that process would be called archiving but am now thinking it's called exporting.

What you want to do is export mail from the account. But I really do not think Thunderbird is as reliable as you need to do this directly to a local folder. Using the export tool to export folders to an mbox file will provider a Thunderbird independent backup/archive. Which can be re-imported to local folders using the same tools reliably

To be sure I don't lose anything, I absolutely want to leave the original folders/emails in my Outlook account

So using a POP account which only downloads to local storage the inbox folder is not going to be an option as you would need to move the mail to have it download.

until I've copied them to local folders and verified everything came through. Once I'm sure the emails are safely in a local folder, I'll delete most of them from the Outlook server.

See my comments on the import export add-on.

From my very, very short experience with TB, it looks like "archiving" means moving (not copying) items from one folder to another but the folder receiving the items still resides in TB, not on my PC. Is this correct or am I mistaken?

You need to start thinking in terms of "account" and most certainly the default archiving means moving mail around in the account not outside it.

I also think that "exporting" means moving (not copying) items from one folder to another folder that may, or may not, be a local folder on my PC.

Export is always a copy. It moves nothing.

I think the choice of folder and location is mine during the export process. Is this correct?

Only if you use the addon. File save really only works for a few emails.

And is it correct that the items are permanently removed from their original folder during exporting?

Most emphatically not.

While playing around with one of the email accounts I successfully moved to TB, I selected a group of 10 emails in a particular folder, right-clicked on the group, and selected "Copy to ..." where I selected a new local folder. This successfully placed copies of the 10 emails in the local folder while leaving the originals in the folder on TB. I suppose I could use this process to place the emails from my primary account into local folders but it would be time-consuming (unless I could "select all" for a given folder and copy a very large number of emails at one time).

Select all Ctrl+A as well as using the block selection of ctrl key and shift with the mouse selection are all options. As I mentioned earlier I do not recommend using them for bulk moves as the risk of loosing something rises as the number of mails increases. Historically a few hundred would move ok but a few thousand would not. Hence my recommendation of the import export tools addon.

I'd very much like to know if my understanding of archive, export and copy are correct and, if so, which I should use to get my emails into local folders. Thanks in advance.

I think they are not correct.

Archive is primarily moving mail around in an account. It can be configured to place mail in another account i.e. "local folders", but dependent on how the IMAP server works (it does not work at all with gmail) it is best to just consider it to be within the account.

Copy is what it says on the tin. But again silent timeouts can cause issues for large numbers of mail. Folder copies (drag and drop usually) in my experience almost always are incomplete.

Export is not something Thunderbird does natively and requires an addon. the ImportExportTools addon is the de jure tool for this and handles the process by verifying each mail with the IMAP server as part of the export so it is slow and bandwidth heavy.

Thank you for the very detailed and helpful answer. Here are a couple of comments with one or two more questions. 1. I'll start referring to "accounts" as you suggested. 2. I understand the difference between IMAP and POP, and specifically set up both new accounts as IMAP. 3. I added "ImportExportToolNG" to TB after doing research but was waiting to use it until I got a response to my post. 4. "Archiving" mainly moves mail within an account but not outside the account. Not what I want to do. 5. "Exporting" does NOT remove the mail from the original account. Good; that's what I want. I want the original to remain until I decide to delete it. 6. "Copying" does what it says but may be unreliable when used with thousands of emails at one time. Not a risk I want to take. 7. You don't recommend using TB to export folders directly from the original account into local folders. Rather, you recommend using the export tool (ImportExportToolNG) to export folders to an MBOX file and then export the folders from the MBOX file to local folders. Can you point to a post or tutorial that explains what an MBOX file is and how I set it up?

By the way, I've never used an email client before but am impressed with how TB permits me to have several accounts open at the same time. Even when I get most of the emails safely moved from my primary account, I intend to stick with TB.

Thanks again.

OK, I've read up on MBOX in general. I think I understand the concept but I'm missing one or two key steps in making the process work. I have an account in TB that has a folder called Test containing a dozen emails. The account is IMAP. I want to move those emails eventually into a local folder called Test1. You previously suggested that I do NOT move the emails directly from their current account folder into a local folder but instead move them into an MBOX file in TB and then move them into the local folder. Exactly what steps do I take in this simple example to do what you suggested? Thanks.