[oedo]

If you're interested in diagramming system architecture in a top-down fashion, I'll pile on another recommendation to check out the C4 model[0].

As others have mentioned, to effectively communicate a system you must limit the context to particular layers of abstraction, and C4 is a good approach to doing just that. There's also a C4 plugin for PlantUML[1].

But don't forget that, as with all visualizations, audience and purpose are key.

Consider whether you are addressing short-term needs (eg identifying inefficiencies, modeling for a client pitch) or long-term needs (eg knowledge retention, managing complexity). If your audience's needs are short-term, you can certainly get by with much simpler tools (eg Inkscape, excalidraw/draw.io/etc, picture of a whiteboard, doodles on a napkin).

Also consider whether or not you actually have a problem better served by bottom-up (ie generated) visualizations (eg ERDs for database schema refactoring, heatmaps for profiling, GraphViz for debugging DAGs).

[0] https://c4model.com/

[1] https://github.com/plantuml-stdlib/C4-PlantUML