well to start off, you dont even have to use brushes!!(surprise surprise) but if you choose to use them, i dont think it matters if theyre your set or someone elses, for me its the outcome(as long as you didnt rip). and to answer your question about the defaults, you shouldnt be asking how do other people do that but what can i do as an artist to master default brushing, that way youll be more focused on your own improvement rather than someone elses. As filters go, just play with them until you get a cool effect. Pixel2Life also has a lot of filter tutorials, you can look there as well.