This sort of capping is typically applied in order to eliminate the need to re-paint wood window trim.
Caulking and sealant materials may be used to help prevent leaks but these products are not considered reliable in the long-term.
The most common material used in residential window capping is factory painted aluminum.
An alternative to factory painted aluminum is to use a vinyl coated aluminium material.
The quality of paint on some aluminum coil stock begins to chalk or fade in about five years so the claim of being maintenance free can be misleading.