I thought Lisp was suppose to be simple in that: it's all just lists.

"Lisp is simple" is mainly an ignorant meme that is used to belittle the interests of people who work in, or on, some kind of Lisp. There are simple aspects, but the aspects are numerous and the combination is complex.

Lisp has kept bright minds busy for sixty years and continues to do so.

Making a production Lisp from scratch is a pretty big undertaking.