Well, the problem is that this isn’t a NixOS problem; you’re both learning how to use NixOS and how display managers work at the same time (on top of likely a bunch of other things). This isn’t trivial, and even if it was, we’re here to help, don’t apologize!
Check this response I gave to another thread asking a similar question: Trying to setup clock and wallpaper in dwm with the default login manager - #2 by TLATER