First is this likely to happen again - any ideas why.
Macs aren't immune to errors. They're complicated devices, just like any other computer. So, yes, it could happen again.
You say you have 15 years of data. That must be one heck of a big database. Especially if it has a lot of image/video/file attachments. As far as why, there are a multitude of possibilities. Possibly, Outlook was having trouble handling such a large database and finally keeled over while accessing or writing data. The drive has a bad sector where part of the data was recently written and can't be retrieved, causing Outlook to crash, which in turn munged the database. That's just a couple.
do you (sic) mean I need to find the outlook file only to restore or do a complete restore and if the latter will I lose recent emails sent & received.
The version you show would be Outlook 2019 or 365. The database is here:
/Users/your_account/Library/Group Containers/UBF8T346G9.Office
The bolded name is the folder everything to do with Outlook (and some other Office data) is in. You would retrieve that same folder from your backup and replace the damaged one. Do this with Outlook closed.
You will very likely lose all email received and sent since your last backup. The backup you have, of course, will only have in it what was current at the time it was made.
What I would do to start is move the broken UBF8T346G9.Office folder to another location, such as the desktop. I would also make a copy of it to another drive so you have it in more than one place. Once that database is safely stored away, put the backup in the normal location and launch Outlook. You should be able to access everything in that database.
As far as retrieving newer data, it could be rather time consuming. In the copy of the damaged database you put on the desktop, open to this folder:
UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Data/Messages
This is where all of your emails are, regardless of what folder they're in within Outlook's interface (Inbox, Sent, Deleted, etc.). The very big problem is, none of this is any logical order. At all. The most newly dated folder could have an email in it from two years ago.
Open any of the folders. You'll something like this:
AC370D09-3100-4085-A585-7B0FA641D504.olk15Message
Unfortunately, this isn't any help. If you double click any of these files, you'll get a message from Outlook that it is not associated with the default profile. If you double click any of these same types of messages in the database Outlook is currently using, they open right up. So why it won't open these same file types from another location makes very little sense.
All of this means I know where the newer emails are, but no idea how to retrieve them, because Outlook will, apparently, only access emails in the current database. You would have to hope putting the broken UBF8T346G9.Office folder back in the default location will manage to open if you try again.