Last week, I read Wikipedia's article on Alexander Grothendieck. It lists his twelve greatest contributions to mathematics as accounted for in Grothendieck's own Récoltes et Semailles. The final item on this list is

The "schematic" or "arithmetic" point of view for regular polyhedra and regular configurations of all kinds.

A quick search revealed an unanswered MathStackExchange post with the same question, from 12 years ago!

Can anyone elaborate on this statement, or recommend a treatment of this topic?