In a star if helium fuses to make carbon, what makes the elements in between?
The elements Lithium, Beryllium and Boron are made by cosmic rays.
The elements Hydrogen and Helium were made during the Big Bang. Helium is also made by fusion reactions in stars.
The elements Lithium Beryllium and Boron can't be made in any quantity is stars as they are intermediate steps in other fusion reactions. Any of these elements present at the birth of a star will get consumed by later reactions.
These intermediate elements (Lithium, Beryllium and Boron) are produced by cosmic rays. These are high energy protons and other particles. These collide with nuclei of Carbon and Oxygen which breaks them down and produces Lithium, Beryllium and Boron.
Carbon is produced by reactions such as the triple Helium reaction. This reaction explains what happens to Beryllium.
The diagram shows where all of the elements are created.