The code being injected doesn't need to hardcode any absolute addresses of the dependent functions. It works fine when the OS kernel randomizes virtual addresses of all DLLs.

> how injecting a DLL works, compared to what's being done here?

Very similar, the main difference is what's injected.

The OP is injecting code and running it. The injected code needs to be position-independent, in practice this means it needs to be written in assembly, which is very hard to do for non-trivial things.

I normally use VirtualAlloc to allocate a UTF-16 buffer for the path of a DLL I made, then use CreateRemoteThread to run LoadLibrary function.

This way I can use normal C++ with all the features in the code being injected.