The “Pool” is separate for each project, unless you’ve specified a Default Pool in a template.
However, it is only a LIST of files used within that project.
Everything recorded in that project will, by default, be in the Audio folder within the project folder.
But if you imported any files from elsewhere they may NOT be in the Audio folder.
When you import an audio file it asks you if you want to copy it to the Audio folder - and it’s usually a good idea.
References to file locations in the Pool are absolute - including Drive letters and such - so if any of that changes Cubase will not find the files unless they are in that Audio Folder in which case it will just warn you the location has changed.
You can, however, search for the files if you have any idea where they might be. File names - even missing - are listed in the Pool.
You can also Import or Export Pools between projects - like if there’s a bunch of loops or files you use regularly and want them at hand. But, again, this is only Importing or Exporting the LISTS unless you specify to copy to Project directory.
When you backup a Project it will ask to move all files to the New Location - a good idea in most cases. However, it wasn’t always like that. So ancient projects could have lost files by not consolidating properly (Archiving) prior to moving. It’s happened to me.