The stem can be made of various materials:

Mesh :

Mesh fabric for breathability and lightness.

Leather :

An ancestral material offering excellent comfort and support for the foot inside the shoe.

Flyknit (Nike) & Primeknit (adidas):

A strong, lightweight weave that perfectly conforms to the shape of your foot, giving the pair premium comfort. Both materials are often accompanied by TPU (plastic) to reinforce the sneaker in certain areas.

Fuse :

A combination of three materials, including mesh and plastic, for enhanced durability.

Flyweave:

Uniquely woven, seamless threads.

BattleKnit :

A quilted Flyknit offering several levels of density for a snug fit.

Flyknit 360

A Flyknit that goes all the way round the foot to limit unnecessary layering and best fit the shape of the foot.