ref – https://www.englishwithashish.com/adjective-complement-masterclass

An adjective complement is a phrase or a clause that completes the meaning of an adjective by giving more information about it.

note:

phrase – A phrase is a group of words that stand together as a single grammatical unit
clause – subj and verb

• An adjective complement is a phrase or a clause.
• It comes right next to an adjective.
• It sits right next to an adjective.