Octavia E. Butler's 'Xenogenesis' trilogy (now often called 'Lilith's Brood') offers some of the most challenging and biologically-focused worldbuilding imaginable. After humanity nearly destroys itself, we are 'rescued' by the Oankali, a gene-trading alien species.
Butler builds the Oankali biology and culture from the ground up: their three sexes (male, female, ooloi), their sensory tentacles, their deep drive to crossbreed and create new life. The worldbuilding is in the visceral, uncomfortable experience of humans forced to coexist with—and eventually merge with—this utterly alien consciousness. It’s a profound exploration of otherness, consent, survival, and what it means to be human when your DNA is no longer your own. It’s mature in the most unsettling and necessary way.