From what I gathered, they are monsters who are obsessed with figuring humans out. I don’t get why humans don’t ever appear afraid of the monsters. Perhaps they are mind controlling aliens? Also, where is Sesame Street? Is it another planet?

I googled lore for info and came across this very useful information

Elmo – Peter Pan Syndrome

Despite being over 40 years old, Elmo has never matured beyond the level of a small child. Though not officially recognised as a mental illness, Peter Pan Syndrome is often used to describe adult individuals who exhibit childlike behaviour