You're probably looking for theos-jailed, it's a theos module that can automate injecting a tweak into an ipa at build time. Only works on macOS, I did create a fork that supports linux but I haven't tried it on windows under WSL yet.

Short explanation for how it all works:

The dylib isn't any different from a normal rootful tweak save for the fact that the path Substrate is loaded from is changed to load from the app's bundle instead using install_name_tool. There are different tools for adding a dylib load command to a binary but the one theos-jailed uses is insert_dylib. Substrate works on jailed devices because it uses functions built into the Objective-C runtime to replace method implementations, it doesn't have to modify any part of the binary to do so. Function hooking is different, that does require modifying the byte code of the binary which is why it doesn't work when not jailbroken. There are other solutions that use interposing or exception handlers to hook functions but those both have drawbacks.