In the digital attention economy, brands are increasingly utilizing diverse content strategies, including user-generated content (UGC), brand-generated content (BGC), and AI-generated content (AIGC), to engage consumers and shape their brand perception. This research investigates the psychological mechanisms that influence consumer engagement with these three content forms, utilizing cognitive processing theory (CPT). The research insights are based on a survey of 291 respondents, which measured engagement behaviors, emotional responses, and perceived authenticity. Moreover, 17 marketing professionals, through semi-structured interviews, provided insights into content reception, trust dynamics, and platform-specific interaction patterns. Results indicate that UGC produced the highest levels of perceived authenticity and emotional resonance, positively influencing engagement and brand trust. BGC was associated with consistent brand identity but yielded moderate emotional impact. AIGC, while efficient and scalable, generated uncertain responses, valued for personalization yet questioned for credibility. Thematic analysis of the interviews underscored the growing strategic integration of AIGC in marketing while also highlighting consumer sensitivity to transparency and ethical content sourcing. This research contributes to the knowledge of digital marketing by examining how psychological responses to different content origins influence digital brand engagement. Moreover, the practical implications highlight the need for content hybridization strategies that strike a balance between authenticity, control, and scalability.