How do I repair the startup volume by running First Aid from Recovery. I have restared from recovery and chosen disc first aid. It says it has found corruption

I’ve not encountered your problem but I’m going to suggest a possible solution.

By the term corruption I’m presuming that there isn’t a hardware problem so I’m suggesting a system restore from Time Machine backup.

I believe that Mal-S is correct and you're headed for a reinstall of your OS and data from a Time Machine backup.

Before you do anything further, please see these support documents –

How to repair a Mac disk with Disk Utility - Apple Support:

https://support.apple.com/en-us/HT210898

How to erase an Intel-based Mac - Apple Support:

https://support.apple.com/en-us/HT208496

I am not entirely sure that is what they have done.

I don't think they understand the difference between running First Aid from the Boot Drive

using Applications/Utilities/Disk Utility and running Disk Utility> First Aid from the Recovery HD.

Only running Disk Utility > First Aid from the Recovery HD will actually perform the repairs.

Have you booted to your Recovery HD.

Restart the mac while pressing and holding down the Command and R keys.

Your mac will restart to your Recovery HD.

From the Utilities window select Disk Utility, press Continue.

highlight Macintosh HD or the nae you gave your Disk and click on First Aid.

Run it until you get the green tick.

This is exactly what he has said he has done in the opening post.

It sounds like the OP ran First Aid (verification) while booted into the boot drive and it told him to run First Aid from the Recovery volume. And that's what Eau Rouge told him to do.

My bad, I failed to notice that the MacOS was version 10.12.

I'm getting used to the Big Sur Disk Utility which just runs & fixes.