Where is iPad back-up stored?

In April 2017 I had a MacBook Pro (late 2013 version) which would have would have been running whatever was the latest version of OSX at the time.

From time to time I backed up my iPad to the computer via iTunes. I assume that the back-up of the iPad would have been stored somewhere on the computer hard disc.

In that month I made a back-up copy of the computer hard disc onto an external drive which I still have.

I now have the latest MacBook Pro 16 inch which runs the latest software.

I have plugged the external hard-drive back-up into my new computer because I want to find the iPad back-up. I've browsed and searched to no avail.

Maybe I'm looking in the wrong places and/or the file name is something that is not intuitively obvious!

Any guidance on (a) what sort of filename I should be looking for, and (b) where I might find it, would be much appreciated. Thank you.