What is contained in project files?

I intend to uninstall Cubase and reinstall on a different drive. If I save all the project files, will I be able to reopen the project without any loss of data? What exactly does a project file contain? All the audio recordings, track info, plugin info, etc?

No. The audio is not contained in the project files. There are other files saved in the project folder as well.
I think Archive… will put everything together so you can easily move everything.

Actually, check the manual but I think Prepare Archive will get everything together and then you can copy the whole project folder.

Or use Backup Project. Any 3rd party plugins that reference audio files will need to be copied manually. For example, any Native Instruments Kontakt or Battery instances. Save these manually inside your project folder. I create a folder called Other and put all 3rd party plugin files in there. When you move to another system, it will ask you to locate the files. All you need to do is select that folder and it will find them all.

Using the Archive feature created a ‘Pool’. I see the audio files have been stored where I want them. So now I should back up the project and I will have the backup and audio files. Then when I reinstall Cubase I should be able to open the backup project and import the audio?

It should automatically load the files in the pool. If not, it will ask you to locate the files. Once you have located the files and Cubase loads them all correctly, then you should save the session so it remembers the location.