How can you install Windows 7 on a 2017 MacBook Air running macOS Catalina?
Windows 7 has to be installed to BIOS boot. Your 2017 MacBook Air does not have a BIOS in the firmware. Therefore, you can not install Windows 7 on a physical 2017 MacBook Air.

You should be able to install Windows 7 in a virtual machine. VirtualBox happens to be a free product that can be used to run Windows 7 as a virtual machine. Usually the older operating systems run adequately fast in virtual machines on newer Macs. 

The latest version of Windows 7 with SP1 can be downloaded from Download Windows 7 Disc Images (ISO Files). You will need a product key to download. This ISO can be installed in VirtualBox without entering a product key, if you want to test before committing. Of course, after a trial period, you would need to provide a product key. VirtualBox does provide all the required Windows 7 drivers, which can be installed after installing Windows 7.