What happens to local files on iOS when the app is updated?

My iOS app is writing to local files to:

/var/mobile/Containers/Data/Application/A.../Library/a.txt

When my app updates, it gets a new application container ID and folder:

/var/mobile/Containers/Data/Application/B...

What happens to all the files I wrote to container ID A?

Is there an "update hook" that will allow me to copy all the "A" container files to path "B"?

Can the new version of the app (B) read the old versions files (A)?

Is there any documentation around what happens to the filesystem during updates?

Is it possible to recover files from container A after B has been installed?

from What happens to local files on iOS when the app is updated?