Most verbs use either avoir or être as the auxiliary verb in le Passé Composé (or other compound tense), but demeurer uses both, depending on what it means in the sentence.
Demeurer is a peculiar verb, a bit like an old-fashioned version of rester (to stay), and is much less used nowadays.
être + demeuré <adjectif>
= to remain <adjective>
Note: When demeurer is used in this sense (to remain) then it is part of the set that use être as the auxiliary verb (A.K.A. the DR & MRS P VANDERTRAMP verbs or the maison d'être verbs).
avoir + demeuré <quelque part>
= to live <somewhere>
= to stay <somewhere>
When demeurer is used in any other sense (to live or to stay somewhere) then it uses avoir as the auxiliary, like most verbs.
Here is the list of all "two-auxiliary" verbs in compound tenses: