My favorite app for creating diagrams is excalidraw. It is a free, open-source, web application that has all the features necessary to quickly create almost any kind of workflow image. The user interface is terrific and easily grokked by anyone familiar with drawing apps. You can export images in either SVG or PNG formats.
There was only one thing that bothered me about the app. When images are exported, there is no padding or margins around the image, which can make them hard to work with. I wasn’t the only one with the issue and someone opened a github issue to discuss possible solutions. In the end, the issue was closed without a resolution, because there didn’t seem to be a generic solution that would please everyone.