I can testify a demon can reside inside a Christian, frkm recent experience even.
The way to understand it is they can reside in your flesh / sinful flesh and create strongholds. They cannot possess your spirit, that is clean in Christ.
The part of you that can have a demon reside in the sinful nature Paul mentions:
“I do not understand what I do. For what I want to do I do not do, but what I hate I And if I do what I do not want to do, I agree that the law is good. As it is, it is no longer I myself who do it, but it is sin living in me. For I know that good itself does not dwell in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For I do not do the good I want to do, but the evil I do not want to do—this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it.” (Romans 7:15-20).