LibreOffice Draw is probably worth a mention. It's that suite's version of Visio. So, you can draw boxes easily, and it has automatically routing connectors to link them with arrows or lines that stay attached as you drag the boxes around. (So, it's not like Inkscape, which is the freeware version of a program like Corel Draw. LibreOffice calls their tool "Draw", but it's more linked-boxes oriented than Inkscape, despite the name.) I haven't used Draw a lot -- but I have used Visio quite a lot, and it's usually pretty easy to create a diagram with it.
https://www.libreoffice.org/