Hi,
I’m unable to install Xcode 15.0 on my macos catalina, since it requires a macos 13.5 or newer. Is there any solution or alternative for xcode where I can program in swift to develop iOS applications ?
xcode is a beast; it takes a lot of space on one’s Mac. (yikes). 
For free IDEs–Have you looked into IntelliJ or Visual Studio?
Try here for some alternatives.