no os on mac pro 2008

If your MBP came preinstalled with an OS newer than El Capitan you will not be able to do the conversion

from the El Capitan download to the Install OS X El Capitan.app which is necessary

for you to create a bootable USB stick that you then use to resurrect the 2008 MP.

Do you have or can you get access to a Snow Leopard installer DVD, you can buy them on E-Bay.

Then you could install Snow Leopard on your 2008 MP, you would then need to update it to

10.6.8 using this download. https://support.apple.com/kb/DL1399?viewlocale=en_US&locale=en_US

Then check for any other updates in the App Store.

Then you will be able to upgrade it to El Capitan.

How to upgrade to OS X El Capitan – Apple Support

Go to Section 4 and click on Download OS X El Capitan.

This downloads InstallMacOSX.dmg to your Downloads folder.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the .pkg to the Install OS X El Capitan.app which you will find in 

your Applications folder, it should be 6.2GBs in size.

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the InstallMacOSX.dmg

if you need it in future, or you could just make a copy of the Install OS X El Capitan.app prior to installing and moving it to

an external drive for safe keeping. It is also possible to create a bootable USB installer disk, using the Install OS X El Capitan.app

in the Applications folder, and the createinstallmedia command in the Terminal app. Read the instructions here,

How to create a bootable installer for macOS – Apple Support

You don't. If you have the DVD's it shipped with use Install Disc 1 holding the "c" key. Install the system it shipped with and upgrade from there.

If not you need to use another that's capable of El Capitan 10.11. Download it using your Apple ID and create an install disk. You can use DiskMaker X:

https://diskmakerx.com

You could also install Mountain Lion from a DVD, then upgrade to El Capitan online.

https://www.ebay.com/itm/Mac-OS-X-Mountain-Lion-10-8-5-on-Bootable-DVD-for-Installation-Upgrade/124142879719?hash=item1ce77e03e7:g:ua4AAOSwM7JZvBOP

If you have a Time Machine backup drive, it is likely to have a bootable Recovery HD partition on it, that could be used to download and install some version of MacOS.

Macs can typically boot from any appropriate drive, internal or external. (the backup drive from your Catalina Mac may Not work for this.)

Any Mac that is old enough to download El Capitan (2010 and up), If your MBP shipped with High Sierra or newer I don't think it will download but you can try:

Download OS X El Capitan