As this is turning into a frequently asked question, the following lists the various tools we use to create our illustrations.
Inkscape is used for creating the base images.
We limit ourselves to the fonts in Google Fonts for standardization and universal availability in all platforms.
For most animated illustrations, we coded them in the standard web stack: HTML, CSS, and Javascript. We use puppeteer to capture the results frame-by-frame; then, we use FFmpeg to generate the video, usually using the base images generated by Inkscape as the base.
We use the d3.js Javascript library for the animations and many other things. We also use the same library for generating map projections and other geo calculations.
For animations of stars, we use Stellarium. It has a built-in Javascript engine that we use to generate frame-by-frame screenshots. Then, we use FFmpeg to combine them into a video.
We arrange all of our work in several git repositories so that our contributors can work together. We store our repositories in GitLab.
We use ImageMagick to convert PNGs from Inkscape to JPEGs and other more specific tasks in various illustrations.
We created a build script using Makefiles to automate and streamline our work. It automatically runs all the other commands to generate the output, rarely requiring manual intervention from our contributors.