Any vector drawing tool will let you create a diagram like the example you posted. Examples: Inkscape, Adobe Illustrator, Sketch (Mac only), Affinity Designer, Figma (browser-based with an Electron desktop client).
You'll have to design and style the diagram yourself though. With a vector drawing app, there are no constraints on how the diagram could look - the appearance is entirely up to you.
Dedicated diagramming programs like Omnigraffle (Mac only) and Visio (Windows only) come with predefined diagramming shapes and the ability to connect shapes with lines. They save time and include options to customise the appearance of the diagram. However, the finished diagram may be a little less visually attractive if you rely on the default settings.
Thanks for mentioning Visio, it is now available on the browser as well, for basic, business, flowcharts, network, and cross-functional flowcharts. $5/per user/per month (annual plan)
https://products.office.com/en-us/visio/visio-online-plan-1
Disclosure: I work for the Visio team, so if you have any feedback pls lmk.
I have to say I'm not really experienced, but where do you find such icons, such that they can be simply imported into whichever (preferrably open source) vector graphics software?
You'll have to design and style the diagram yourself though. With a vector drawing app, there are no constraints on how the diagram could look - the appearance is entirely up to you.
Dedicated diagramming programs like Omnigraffle (Mac only) and Visio (Windows only) come with predefined diagramming shapes and the ability to connect shapes with lines. They save time and include options to customise the appearance of the diagram. However, the finished diagram may be a little less visually attractive if you rely on the default settings.