They are copied from http://sqlite.org/arch.html, which is built from (see http://sqlite.org/download.html) data in Fossil at https://www3.sqlite.org/cgi/docsrc/timeline, which leads me to e.g. https://www3.sqlite.org/cgi/docsrc/artifact/d22a2c9642d584b8, which seems to contain the right strings, so presumably is the file describing https://www.sqlite.org/images/arch2.gif and its copy on https://cstack.github.io/db_tutorial/assets/images/arch2.gif

That's fig source. So, I guess this was made by some GUI that uses fig as its native file format. My guess would be xfig (http://mcj.sourceforge.net)