How to buy Logic Pro on a macbook air 2015 that does not support MAC os Ventura?

First you have to buy Logic by login into the appstore with your Apple ID. You may have to use a modern MacOS (from a new user account of friends computer). There are many options. Please make a google search.

After you have bought Logic, you can follow the steps in the video.

How to download older versions of Logic Pro X and Garage Band.

I think YoYoBen's method is actually a work around. You'll need to find someone that is running at least Ventura 13.5. Open the App Store. Log in with your user ID and purchase Logic. I'm not sure if it will try to download right away but if it does, I believe you can cancel the download.

Log out of the App Store for the newer computer.

Now go to your computer. I tried this on Mojave 10.14.6.

Open App Store. Login if you haven't. Select your account on the bottom left. Look for Logic Pro on your purchases. Do not click on Logic Pro itself. Click on the download symbol. It will bring up some message asking if you want to download the last compatible version. Click download and you should be good to go.

There isn't a solution other than to buy a newer mac. You must have at least Mac OS 13.5

Here is a link showing what macs will work.

macOS Ventura is compatible with these computers - Apple Support

Once logged into the App Store, you're able to download the latest supported version that is compatible with your Mac model. You can't select older versions, software really never worked that way. Hardly anyone offers outdated versions of their major products.

There must be a way. I'm in a similar boat, Sonoma is entirely unworkable with my outboard equipment, despite Ventura and Monterey working 100% seamlessly.

if you have ventura 13.5 you can download the latest version of Logic

I don't want the latest, it aint the greatest for me.

Specifically what outboard hardware? Have you verified with the manufacturers that they have firmware updates for macOS 14?