You have said "local file" and "local folder" but I think we need to be clear what you mean or want.

Thunderbird has its own Local Folders account which is local (that is, stored on your local hard disk or equivalent) and so (in part) meets your requirements. But if you mean you want to save messages as files in your file system then you need to export them from your email client.

I can't imagine why anyone would want to store email outside an email client. You lose the benefits of threading, context sensitivity, searchability. If you really want to store messages as files in your computer's file system, you'll need to create a filter and I think you'll need the FiltaQuilla add-on to automatically save them. This offers a "save message as file" option, but if they are saved as eml files, you're really not much better off. You'll still need an email client to open and read these eml files.

Yes, I meant Thunderbird Local Folder. How do I set up a Thunderbird Local Folder account?

When you first run Thunderbird, a default 'profile name' folder is created and so is default 'the 'Local folder' mail account.

'Local Folders' mail account be default also contains the 'Outbox' folder used by all mail accounts.

You then add mail accounts, one for each email address. Those mail accounts could be either POP or IMAP.

How many mail accounts do you currently have in Thunderbird? Are they IMAP or POP mail accounts?