how do i connect my mac pro tower to my 2020 mac book air so that i can use the book as a monitor?

You can use Screen Sharing.

Share the screen of another Mac - Apple Support

There is no direct way.

I think the tower is too old for AirPlay.

lllaass has given you good advice.

Airplay requires a hardware assist, first implemented in Intel Integrated graphics, that the Mac Pro does not include. If that were not enough, its standard Wi-Fi and Bluetooth are antique by current standards, and can not be used for this sort of thing.

Then there is the issue that the MacBook Pro does not have a direct display INPUT of any description, so the MacBook Pro can not be used "as if it were a display".

Take the disk that has the photos out of the Mac and put/attached it to an USB dock/enclosure compatible with the disk and attached it to your MacBook and it temporarily move them to your MacBook so you can later move them to external drive or attached oth external drive to the MacBook and copy from one disk to the other.

...So screen sharing (the screen of the Mac Pro appears in a WINDOW under MacOS running on the MacBook Pro) is your only option.

Got a TV set? Mac Studio can support a TV set as an external display, using its HDMI port.