In some romance anime, there are red strings around the MCs' body parts, say hands. Why?
This is based on a common East Asian belief known as the "Red Thread of Fate", or "Red String of Fate". It is said that soulmates are bound together by an invisible red string, with one end tied around the male's finger, and the other end tied around the female's finger; and that those soulmates are destined to meet, no matter what.
Look closely at the second Koi to Uso screenshot and you'll see that the strings are indeed tied around their little fingers. The red strings in Quintessential Quintuplets don't appear to be tied to anyone, which likely symbolises the fact that we know the protagonist is destined to marry one of the quintuplets, but we don't know which one.