Question

Structually, what do flagella and cilia have in common

Answer 1

Flagella and cilia are structures made up of microtubules.  They are covered by an extension of the plasma membrane and project from a cell to aid in motility.

Answer 2

 They are both projections from the cell and are made up off microtubules (collectively are called axoneme). They both have the same internal structure, but the major difference between the two is their length.