New to peptide research — where to start?

Just discovered this community. Background in molecular biology but never worked with peptides specifically. The compound database here is incredible.

What would you consider the most well-researched compounds for a beginner to start reviewing? Looking for things with strong clinical trial data, not just rodent studies.

Also, how critical is the COA verification when selecting a research supplier? I see some vendors listing third-party testing and others not mentioning it at all.