Did knight wear chain mail under plate armor?

yes

Not always in the early stages of Plate it was nessercary since the armourer could not cover most of the body.

However when we reach the Gothic and then later Maximillian and Grenwich style armour the chain becomes useless since there were so few openings in the plate that wearing chain would only slow the wearer down, there are even examples of late period armour where there are no openings what-so-ever and every plate overlaps on itself.