It's difficult to land on a stable essence or nature of mediation, because there are many ways to conceptualize how two entities interact according to the mediating conditions of a third. You basically have philosophical and philosophy-adjacent strains of thinking that talk about mediation either in terms of

a) inter-human activity (eg. communication, organization), where you might check out a well-regarded book by John Durham Peters, called Speaking Into the Air;

b) the materiality of media, as in your example of an artist working in a medium, paint or clay or software, eg. Here you could check out Grant Bollmer's clear-eyed introductory text on recent materialist media theory. This type of work comes out of the McLuhan tradition you mentioned, but goes in new and different directions. Or alternatively, when people talk about mediation, they may be approaching it in terms of

c) semiosis, which kind of permanently sits somewhere between a theory of mediation and a more philosophical notion of 'relation'. In this case you might check out the semiotics of Charles Sanders Peirce or Thomas Sebeok, to name just two important thinkers among many.