Usually made from silk, with sometimes impressive patterns.
It has a 半襟（はん えり）, neckband, which are normally loosely stitched on the 長襦袢. They are meant to be removed and soaked in soapy water to be washed.
These 半襟 are supposed to match the 着物.
However, you only see the collar of the 長襦袢, so instead, you could use a 伊達衿（だて えり）, which is just the collar, and which is stitched directly onto the 着物. In that case, you could omit the 長襦袢, and just wear 肌襦袢 directly under the 着物 (in summer, for example).
These days you can use a 重ね衿ピン, a pin, to attach either the 伊達衿 to the 着物 or the 半衿 to the 肌襦袢.
There are also cotton T-shirts with a polyester 半衿 attached. These seem quite convenient. (Search for: 半襦袢 うそつき 襦袢 袷用 日本製 男物 塩瀬 洗える tシャツ 半襦袢 塩瀬 半衿付き 肌襦袢 筒袖 共袖 衣紋なし メンズ)