The purpose of this study is to measure the difference in time to developing or worsening memory, thinking, or functional problems due to Alzheimer's disease occurring in participants receiving study drug compared to placebo. Participation could last up to 255 weeks including screening, a double-blind treatment period, and a double-blind observation period. In addition, eligible participants who receive placebo during the double-blind treatment period may choose to extend their study participation to receive open-label remternetug in an extension period.