Lying = 躺 [Tǎng]

Someone is lying flat on the sofa with their head under a glass table, watching the phone that rests on the glass above their face. That flat-out position is 躺.

躺 (tǎng) means to lie, as in lying on your back or your side. It is about body position and has nothing to do with telling a lie. 躺下 (tǎng xià) means to lie down.

我想躺一会儿。 — Wǒ xiǎng tǎng yíhuìr. — I want to lie down for a while.

他躺在沙发上。 — Tā tǎng zài shāfā shang. — He is lying on the sofa.