I'm a big fan of sequence diagrams, especially those that can be generated from markdown-like syntax. I use Lucidchart at work, but there are a few free options as well -- in this comment I'm trying to advocate for the concept rather than a specific tool.
Sequence diagrams, to me, give the benefits of a flow chart, but with a much tighter explanation of the actual flow and relationships. You can read them from the upper left corner to the lower right, just like (western) writing. You get not just the entities involved, and not just the way they are connected, but the _ordering_ of those connections, which I find super helpful. The concept is simple enough that I can hand draw it on a whiteboard or scrap of paper, and for electronic formats, if you generate these from markdown you now have the same data in both a visual and textual format (handy for folks who have a harder time accessing pure visual presentations), and auto-generation from text makes it easy to update and so more likely to maintain as the system changes.
Sequence diagrams, to me, give the benefits of a flow chart, but with a much tighter explanation of the actual flow and relationships. You can read them from the upper left corner to the lower right, just like (western) writing. You get not just the entities involved, and not just the way they are connected, but the _ordering_ of those connections, which I find super helpful. The concept is simple enough that I can hand draw it on a whiteboard or scrap of paper, and for electronic formats, if you generate these from markdown you now have the same data in both a visual and textual format (handy for folks who have a harder time accessing pure visual presentations), and auto-generation from text makes it easy to update and so more likely to maintain as the system changes.