Almost every result in longevity and peptide science starts at one of three levels, and the level changes what the result can tell you.
Cell studies (in vitro). Researchers expose cells in a dish to a compound and measure what changes: gene expression, cell survival, markers of aging. This is fast, cheap and useful for narrowing down what's worth testing further. It's also the furthest result from a human body: a dish has no circulation, no immune system, no organs interacting with each other, and no dose that has to survive digestion or metabolism to reach the cell.
Animal studies (in vivo). The same compound, tested in a living organism, usually mice or rats for mammalian biology, sometimes simpler organisms like worms or flies for fast, large-scale screening. This adds a real biological system, but animal and human metabolism, lifespan and disease biology diverge in specific, well-documented ways.
Human trials. The only level that directly answers "does this work in people," and the most tightly regulated: typically phased, starting with safety in small groups before moving to efficacy in larger ones. A compound can succeed at the cell and animal level and still fail here, which happens often enough that cell or animal results alone are never treated as proof of a human effect.
The practical skill is reading past the headline to the level: "shown to extend lifespan" means something very different if the subject was a worm than if it was a person in a completed phase 3 trial. BioBonds labels every article by the level of evidence behind it for exactly this reason.