home-manager manages your whole user's environment & desktop.

devenv does not do any user-level change (you will not be able to make it configure your WM), but works at the directory level.

For instance I'm currently working on a Rust + C++ project, and my devenv, whenever I enter this project folder: make CMake/g++/cargo/cbindgen available, enable a couple scripts to longer CMake invokations, set-up everything required for C++ and Rust LSPs, and create a couple git hooks to validate formatting etc.