'Nesting' is the act of intentionally settling into your home — tidying, decorating, cooking, doing laundry, creating comfort. In current youth usage it overlaps with 'cozymaxxing' and 'slow living'. (The older meaning of 'nesting' describes pregnant women preparing for a baby's arrival.) If your teen is 'nesting' on a Sunday, they are recharging at home. Harmless and healthy.