I have keys but no locks. I have space but no room. You can enter, but can’t go outside

I start where I finish and finish where I start. You’ll find me on a racetrack, in code, or on a DJ’s set.

You thank for me beneath the Christmas tree, I hope you can save me from myself. Add one word, and I'll help you when you're stranded, become me to reach the highest shelf.

Have letters but is not a book Have picture but is not a wall Is round but is not a wheel

At a story’s end, I patiently dwell, But inside your body, I might raise hell. Read me or cut me, either one clicks; I'm found in your books and biology’s mix.