Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
Webgraphviz - WebGraphviz is Graphviz in the Browser
draw.io - Online diagramming application
Penpot - Design freedom for teams
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Graphviz - Graphviz is open source graph visualization software. It has several main graph layout programs.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.