Look Immediate Mode UIs when you have some time to kill. It's a famous paradigm in videogame UI development, so unfortunately most publicly available IMGUI libraries are tailored for use in game engine editor tooling (not very style-able, simple layout algorithms and assumption that they're running in a 60FPS loop), but they're still a good place to look at, to get a general idea. The most famous library is Dear ImGui [1]
The biggest advantage of the immediate mode approach is that you get UI/state syncing for free without having to manage any callbacks/getters/setters/magic properties. It introduces new problems (widgets are now defined implicitly and you need a system for keeping track of them over time), but they're way easier to solve than the UI/state sync issue.
Examples of real-non-video-game software that uses it are File Pilot [2] and RAD Debugger [3], which are modern desktop UI applications with all the modern desktop UI application bells and whistles.
[1] https://github.com/ocornut/imgui [2] https://filepilot.tech/ [3] https://github.com/EpicGamesExt/raddebugger