I use draw.io at
https://app.diagrams.net/It is free and online. I can draw diagrams that I used to draw with Visio but I don't have to install software and can use it from anywhere.
I can export the documents to my local machine as a master file (e.g. file type is *.drawio) or can export them as an image (e.g. JPG, PNG, etc.)
There are lots of subjects that you can turn on (also free) ranging from architecture to electronics.
I can usually find what I need or something that works well enough for my simple needs.
I can also use it kinda like a white-board in screen-sharing meetings.
Though - looking at other comments - yEd looks pretty powerful. I'll have to check it out.