Are there sites or resources that contain step-by-step derivations of mathematical expressions or concepts (e.g. the convolution integral) like it's done here https://en.wikipedia.org/wiki/Convolution#Visual_explanation?
Don't suggest ProofWiki since it's slightly different.
On
Yes there is no database of such things as said by stella but there are always different resources or sites (not a single one) certainly for the different elementary concepts and they can explain or visualize things.
Also there are many videos from different universities or institutions which provide a good explanation of concepts which can be reffered.
If you don't find such a site or resource which could help you then you ask questions here.
There is no database of such things. In general, visual motivations for proofs and concepts are useful, but can be very hard to clearly create. If you have a specific example in mind, I would recommend making a post asking for a visual explanation of the concept.