is copying someone else's art and selling it is a copyright violation?
Usually. Most non-trivial drawings are protected by copyright law.
Some drawings are sufficiently basic that either they are no capable of being protected by copyright law, or are in the public domain because they've been used for a long time (e.g. the Jesus fish symbol has been in use since sometime in the 100s CE).
Let's say someone just puts the drawing made on a popular TV show on a
t shirt.. Is that violating copyright laws
Usually, the person who makes the drawing is the author for copyright purposes and would violate copyright protections against copying.
Let's say someone makes a drawing of a drawing made on a popular TV
show then puts it on a t shirt and sells it.. Is that violating
copyright laws?
Usually, the person who makes the drawing is the author for copyright purposes and would violate copyright protections against creating derivative works.
I'm using "weasel words" like "most" and "usually" because the fact pattern given in the question is very thin and doesn't provide all of the facts relevant to the legal question presented. We know very little about the drawing or about how true the copy of the drawing is. A two year old's attempt to copy a drawing that is a realistic Presidential portrait made in 1990 is probably not true enough to constitute a derivative work, even if it was literally made from observations of that portrait. We also know very little about the context in which the underlying drawing appears. It the drawing is a federal highway sign or a scientific diagram or a painting from the 1500s, it might not be protected at all. If it is a portrait featured on a documentary about a local artist, it probably is protected by copyright law.