How do you build the IPA without Xcode?

Hell, whatever that process is, I might even use it in a Mac (if I had one), just because I have Xcode so much.

If the actual goal is to sideload—which is what was being discussed—you don’t need to compile anything. You’d download a prebuilt but unsigned IPA, and then use Cydia Impactor to sign it with your developer certificate.

If you're ok trusting a third party (such as an alternative app store), they can build it, send you the binary, you sign it (on Windows or Linux), and install it on the phone.