Ask HN: What tool does professional software engineers use to draw UML diagrams?

first of all I would recommend to stay away from UML. It's a bloated tool. that don't translate to code.

for the design phase I think the C4model and a sequence diagram provide all the tools to think about your project.

in order to produce such diagrams you can use plantuml. Because that way you can store it with your code in git.

Check out Excalidraw - https://excalidraw.com/ It is an open source, no-frills tool that you can use to quickly draw simple diagrams.

I am using it quite often to sketch LoFi mocks.

It stores in your browser's localstorage, and you can easily collaborate with others in realtime.